Section 1. We will now study the adult anatomy of the frog, and throughout we shall make constant comparisons with that of the rabbit. In the rabbit we have a distinctly land-loving, burrowing animal; it eats purely vegetable food, and drinks but little. In the frog we have a mainly insectivorous type, living much in the water. This involves the moister skin, the shorter alimentary canal, and the abbreviated neck (Rabbit, Section 2) of the frog; the tail is absent-- in a fish it would do the work the frog accomplishes with his hind legs-- and the apertures which are posterior in the rabbit, run together into one dorsal opening, the cloaca. There is, of course (Rabbit, Section 4), no hair the skin is smooth, and an external ear is also absent. The remarkable looseness of the frog's skin is due to great lymph spaces between it and the body wall.

Section 2. If we now compare the general anatomy of the frog (vide Sheet 11) with that of the rabbit, we notice that the diaphragm is absent (Rabbit, Section 4), and the body cavity, or coelom, is, with the exception of the small bag of the pericardium round the heart, one continuous space. The forked tongue is attached in front of the lower jaw, and can be flicked out and back with great rapidity in the capture of the small insects upon which the frog lives. The posterior nares open into the front of the mouth-- there is no long nasal chamber, and no palate, and there is no long trachea between the epiglottis and the lungs. The oesophagus is less distinct, and passes gradually, so far as external appearances go, into the bag-like stomach, which is much less inflated and transverse than that of the rabbit. The duodenum is not a U-shaped loop, but makes one together with the stomach; the pancreas lies between it and the stomach, and is more compact than the rabbit's. There is no separate pancreatic duct, but the bile duct runs through the pancreas, and receives a series of ducts from that gland as it does so. The ileum is shorter, there is no sacculus rotundus, and the large intestine has no caecum, none of the characteristic sacculations of the rabbit's colon, and does not loop back to the stomach before the rectum section commences. The anus opens not upon the exterior, but into a cloacal chamber. The urinary and genital ducts open separately into this cloaca, and dorsally and posteriorly to the anus. The so-called urinary bladder is ventral to the intestine, in a position answering to that of the rabbit, but it has no connection with the ureters, and it is two-horned.

Section 3. The spleen is a small, round body, not so intimately bound to the stomach as in the rabbit, but in essentially the same position.

Section 4. Much that we knew of the physiology of the frog is arrived at mainly by inferences from our mammalian knowledge. Its histology is essentially similar. Ciliated epithelium is commoner and occurs more abundantly than in the rabbit, in the roof of the mouth for instance, and its red blood corpuscles are much larger, oval, and nucleated.

Section 5. The lungs of the frog are bag-like; shelves and spongy partitions project into their cavities, but this structure is much simpler than that of the rabbit's lung, in which the branching bronchi, the imperfect cartilaginous rings supporting them, alveoli, arteries and veins, form together a quasi-solid mass.

Section 6. The mechanism of respiration is fundamentally different from that of the mammal. The method is as follows:-- The frog opens its anterior nares, and depresses the floor of the mouth, which therefore fills with air. The anterior nares are then closed, and the floor mouth rises and forces the air into the lungs-- the frog, therefore, swallows its air rather than inhales it. The respiratory instrument of the rabbit is a suction pump, while that of the frog is a "buccal force pump."

Section 7. The heart is not quadrilocular (i.e., of four chambers), but trilocular (of three), and two structures, not seen in Lepus, the truncus arteriosus and the sinus venosus, into the latter of which the venous blood runs before entering the right auricle, are to be noted. The single ventricle is blocked with bars of tissue that render its interior, not an open cavity, but a spongy mass. Figure 2, Sheet 11, shows the heart opened; l.au. and r.au. are the left and right auricles respectively; the truncus arteriosus is seen to be imperfectly divided by a great longitudino-spiral valve (l.s.v.); p.c. is the pulmo-cutaneous artery -going to the lungs- [supplying skin and lungs]; d.ao., the dorsal aorta [furnishing the supply of the body and limbs]; and c.a. the carotid artery going to the head; all of which vessels (compare Figure 1) are paired.

Section 8. It might be inferred from this that pure and impure blood mix in the ventricle, and that a blood of uniform quality flows to lungs, head, and extremities; but this is not so. The spongy nature of the ventricle sufficiently retards this mixing. It will be noted that the opening of pulmonary arteries lies nearest to the heart, next come the aortic and carotid arches, which have a common opening at A. Furthermore, at c.g.l. [the carotid artery, repeatedly divides to form a close meshwork of arterioles, the carotid gland, forming a sponge-like plug in this vessel.] is a spongy mass of matter, the carotid gland inserted upon the carotid. Hence the pulmonary arteries yawn nearest for the blood, and, being short, wide vessels, present the least resistance to the first rush of blood-- mainly venous blood for the right auricle. As they fill up, the back resistance in them becomes equal, and then greater, than the resistance at A, and the rush of blood, now of a mixed quality passes through that aperture. It selects the dorsal aorta, because the carotid arch, plugged by the carotid gland, offers the greater resistance. Presently, however, the back resistance of the filled dorsal aorta rises above this, and the last flow of blood, from the ventricular systole-- almost purely oxygenated blood for the left auricle-- goes on towards the head.

Section 9. At the carotid gland the carotid artery splits into -an- [a] -external carotid- [lingual] (e.c.), and a deeper internal carotid. The dorsal aorta passes round on each side of the oesophagus, as indicated by the dotted lines in Figure 2, Sheet 11, and meets its fellow dorsal to the liver. Each arch gives off subclavian arteries to the limbs, and the left, immediately before meeting the right, gives off the coeliaco-mesenteric artery [to the alimentary canal]. This origin of the coeliaco-mesenteric artery a little to the left, is the only asymmetry (want of balance) in the arterial system of the frog, as contrasted with the very extensive asymmetry of the great vessels near the heart of the rabbit. [Posteriorly the dorsal aorta forks into two common iliac arteries (right and left) supplying the hind limbs.]

Section 10. Figure 3 gives a side view of the frog, to display the circulation.

{Lines from Second Edition only.}
[The venous return to the heart, as in the rabbit, is by paired venae cavae anteriores and by a single vena cava inferior. The factors of the anterior cava on either side are an external jugular (ex.j.) an innominate vein (in.v.) and subclavian (scl.v.). The last receives not only the brachial vein (b.v.) from the fore limb, but also a large vein bringing blood for the skin, the cutaneous (p.v.). The innominate vein has also two chief factors, the internal jugular (l.i.j.v.) and the subscapular (s.s.v.). The blood returns from each hind limb by a sciatic (l.sc.) or femoral (f.m.) vein, and either passes to a renal portal vein (l.r.p.), which breaks into capillaries in the kidney, or by a paired pelvic vein (l.p.v. in Figures 1 and 3) which meets its fellow in the middle line to form the anterior abdominal vein (a.ab.v.) going forward and uniting with the (median) portal vein (p.v.) to enter the liver.]

-The vessels are named in the references to the figure, which should be carefully copied and mastered. Here we need only- [Comparing with the rabbit, we would especially] call attention to the fact that the vena cava inferior extends posteriorly only to the kidney, and that there is a renal portal system. The blood from the hind limbs either flows by the anterior abdominal vein to the portal vein and liver, or it passes by the renal portal vein to the kidney. There the vein breaks up, and we find in the frog's kidney, just as we find in the frog's and rabbit's liver, a triple system of (a) nutritive arterial, (b) afferent* venous and (c) efferent** venous vessels.

* a, ad = to;
** e, ex = out of.

{This Section missing from Second Edition.}
-Section 11. It is not very improbable that the kidney of the frog shares, or performs, some of the functions of the rabbit's liver, or parallel duties, in addition to the simply excretory function. Since specialization of cells must be mainly the relatively excessive exaggeration of some one of the general properties of the undifferentiated cell, it is not a difficult thing to imagine a gradual transition, as we move from one organism to another, of the functions of glands and other cellular organs. It is probable that the mammalian kidney is, physiologically, a much less important (though still quite essential) organ than the structures which correspond to it in position and development in the lower vertebrate types.-

Section 12. The lymphatic system is extensively developed in the frog, but, in the place of a complete system of distinctly organized vessels, there are great lymph sinuses (compare Section 1). In Figure 5, Sheet 12, the position of two lymph hearts (l.h., l.h.) which pump lymph into the adjacent veins, is shown.

Section 13. The skull of the frog will repay a full treatment, and will be dealt with by itself later. The vertebral column (Sheet 12) consists of nine vertebrae, the centra of which have faces, not flat, but hollow in front (pro-coelous), and evidently without epiphyses (compare the Rabbit). The anterior is sometimes called the atlas, but it is evidently not the homologue of the atlas of the rabbit, since the first spinal nerve has a corresponding distribution to the twelfth cranial of the mammal, and since, therefore, it is probable that the mammalian skull = the frog's skull + one (or more) vertebrae incorporated with it. Posteriorly the vertebral column terminates in the urostyle, a calcified unsegmented rod. The vertebrae have transverse processes, but no ribs.

Section 14. The fore-limb (Figure 6, Sheet 12) consists of an upper segment of one bone, the humerus, as in the rabbit; a middle section, the radius and ulna, fused here into one bone, and not, as in the mammalian type, separable; of a carpus, and of five digits, of which the fourth is the longest. The shoulder girdle is more important and complete than that of the higher type. There is a scapula (sc.) with an unossified cartilaginous supra-scapula (s.sc.); the anterior border of the scapula answers to the acromion. On the ventral side a cartilaginous rod, embraced by the clavicle (cl.) (a membrane bone in this type), runs to the sternum, and answers to the clavicle of the rabbit. In the place of the rabbit's coracoid process, is a coracoid bone (co.), which reaches from the glenoid cavity to the sternum; it is hidden on the right side of Figure 6, which is a dorsal view of the shoulder girdle. There is a pre-omosternum (o.st.) and a post-omosternum, sometimes termed a xiphisternum (x.).

Section 15. Figure 7 shows the pelvic girdle and limb of the frog. There is a femur (f.); tibia and fibula (t. and f.) are completely fused; the proximal bones of the tarsus, the astragalus (as.), and calcaneum (cal.) are elongated, there are five long digits, and in the calcar (c.) an indication of a sixth. With considerable modifications of form, the three leading constituents of the rabbit's pelvic girdle occur in relatively identical positions. The greatly elongated ilium (il.) articulates with the single (compare Rabbit) sacral vertebra (s.v. in Figure 5). The ischium (is.) is relatively smaller than in the rabbit, and the pubis (pu.) is a ventral wedge of unossified cartilage. The shape of the pelvic girdle of the frog is a wide departure from that found among related forms. In connection with the leaping habit, the ilia are greatly elongated, and the pubes and ischia much reduced. Generally throughout the air-frequenting vertebrata, we find the same arrangement of these three bones, usually in the form of an inverted. Y-- the ilium above, the ischium and pubis below, and the acetabulum at the junction of the three.

Section 16. The uro-genital organs of the frog, and especially those of the male, correspond with embryonic stages of the rabbit. In this sex the testes (T., Sheet 13) lie in the body cavity, and are white bodies usually dappled with black pigment. Vasa efferentia (v.e.) run to the internal border of the anterior part of the kidney, which answers, therefore, to the rabbit's epididymis. The hinder part of the kidney is the predominant renal organ. There is a common uro-genital duct, into which a seminal vesicle, which is especially large in early spring, opens. This is the permanent condition of the frog. In the rabbit, for urogenital duct, we have ureter and vas deferens; the testes and that anterior part of the primitive kidney, the epididymis, shift back into the scrotal sacs, and the ureters shift round the rectum and establish a direct connection with the bladder, carrying the genital ducts looped over them. The oviducts of the female do not fuse distally to form a median vagina as they do in the rabbit. In front of the genital organ in both sexes is a corpus adiposum (c.ad.), which acts as a fat store, and is peculiar to the frogs and toads. The distal end of the oviduct of the female is in the breeding season (early March) enormously distended with ova, and the ovaries become then the mere vestiges of their former selves. The distal end of the oviduct is, therefore, not unfrequently styled the uterus. There is no penis in the male, fertilisation of the ova occurring as they are squeezed out of the female by the embracing fore limbs of the male. The male has a pad, black in winter, shown in Figure 1, which is closely pressed against the ventral surface of the female in copulation, and which serves as a ready means of distinguishing the sex.

Section 17. The spinal cord has a general similarity to that of the rabbit; the ratio of its size to that of the brain is larger, and the nerves number ten pairs altogether. The first of these (sp. 1, in Figure 2, Sheet -12- ) {First Edition error.} [13] corresponds in distribution with the rabbit's hypoglossal nerve, a point we shall refer to again when we speak of the skull. The second and third constitute the brachial plexus. The last three form the sciatic plexus going to the hind limb.

Section 18. The same essential parts are to be found in the brain of both frog and rabbit, but in the former the adult is not so widely modified from the primitive condition as in the latter. The fore-brain consists of a thalamencephalon (th.c. and 1), which is exposed in the dorsal view of the brain, and which has no middle commissure. The cerebral hemispheres (c.h.) are not convoluted, do not extend back to cover parts behind them, as they do in the rabbit, and are not connected above the roof of the thalamencephalon by a corpus callosum. Moreover, the parts usually regarded, as the olfactory lobes (rh.) fuse in the middle line. The mid-brain gives rise to the third nerve, and has the optic lobes on its dorsal side, but these are hollow, and they are not subdivided by a transverse groove into corpora quadrigemina, as in the rabbit. In the hind-brain the cerebellum is a mere band of tissue without lateral lobes or flocculi, and the medulla gives origin only to nerves four to ten; there is no eleventh nerve, and the hypoglossal is the first spinal-- from which it has been assumed that the rabbit's medulla equals that of the frog, plus a portion of the spinal cord incorporated with it. The hypoglossal is very distinctly seen on opening the skin beneath the hyoid plate.

Section 19. The first, second, third, and fourth cranial nerves of the frog correspond with those of the rabbit in origin and distribution. So do five, six and eight. The seventh nerve forks over the ear-drum-- the larger branch emerging behind it and running superficially, as shown in Figure 4. There is also a deeper palatine branch of VII. (P.) running under V2 and V3 below the orbit, and to be seen together with V1 and V2 after removal of the eyeball. The ninth nerve similarly forks over the first branchial slit of the tadpole, and evidence of the fork remains in the frog. It is seen curving round anterior to the hypoglossal nerve, and lying rather deeper in dissection. The vagus (tenth) nerve is distributed to heart, lungs, and viscera-- in the tadpole it also sends for forking branches over the second, third, and fourth branchial slits. It lies deeper than IX., and internal to the veins, and runs close beside the cutaneous artery. Most of these nerves are easily dissected and no student should rest satisfied until he has actually seen them.

Section 20. The sympathetic chain is closely connected with the aorta. It is, of course, paired, and is easily found in dissection by lifting the dorsal aorta and looking at its mesentery. In the presence of ganglia corresponding to the spinal nerves, and of rami communicantes, it resembles that of the rabbit.

Section 22. We have already given a description of the mammalian skull, and we have stated where the origin of the several bones was in membrane, and where in cartilage; but a more complete comprehension of the mammalian skull becomes possible with the handling of a lower type. We propose now, first to give some short account of the development and structure of the skull of the frog, and then to show briefly how its development and adult arrangement demonstrate the mammalian skull to be a fundamentally similar structure, complicated and disguised by further development and re-adjustment.

Section 23. Figure 1,I. Sheet 14, shows a dorsal view of a young tadpole cranium; the brain has been removed, and it is seen that it was supported simply upon two cartilaginous rods, the trabeculae cranii (tr.c.). Behind these trabeculae comes the notochord (n.c.), and around its anterior extremity is a paired tract of cartilage, the parachordals (p.c.). These structures, underlying the skull, are all that appear[s] at first of the brain box. In front, and separate from the cranium, are the nasal organs (n.c.); the eyes lie laterally to the trabeculae, and laterally to the parachordals are two tracts of cartilage enclosing the internal ear, the otic capsules.

Section 24. Figure 1, II., is a more advanced, phase of the same structures. The trabeculae have met in front and sent forward a median (c.t.) and lateral parts (a.o.) to support the nasal organs. They have also flattened, out very considerably, and have sent up walls on either side of the brain to meet above it and form an incomplete roof (t.) over it. The parachordals have similarly grown up round, the hind-brain and formed a complete ring, the roof of which is indicated, by b. Further, the otic capsules are fusing with the brain-case. With certain differences of form these elements-- the trabeculae, the parachordals, and the otic capsules, are also the first formed structures of the mammalian cranium.

Section 25. In Figures 1,I. and II., there appears beneath the eye a bar of cartilage (p.p.), the palato-pterygoid cartilage, which is also to be seen from the side in Figures 8,I. and III. It will be learnt from these latter that this bar is joined in front to the cranium behind the nasal organ, and behind to the otic capsule. The cartilaginous bar from the palato-pterygoid to the otic capsule is called the quadrate, and at the point of junction, at the postero-ventral angle of the palato-pterygoid, articulates with the cartilaginous bar which is destined to form the substratum of the lower jaw-- Meckel's cartilage (M.c. in Figure 8,I.).

Section 26. Figure 2 shows a dorsal view of these structures in a young frog. The parts corresponding to these in 1,II. will be easily made out, but now ossification has set in at various points of this cartilaginous cranium. In front of the otic capsule is the paired pro-otic bone (p.o.); behind it at the sides of the parachordal ring is the paired ex-occipital (e.o.); in front of the cranium box, and behind the nasal capsules, is a ring of bone, the (median, but originally paired) sphenethmoid (s.e.). -A paired ossification appears in the palato-pterygoid cartilage the pterygoid bone (pt.), while- A splint of bone, the quadrato-jugal, appears at the angle of articulation with the lower jaw. These are all the cartilage bones that appear in the cranium and upper jaw of the frog.

Section 27. But another series of bones, developed first chiefly in dermal connective tissue, and coming to plate over the cranium of cartilage, are not shown in Figure 2. They are, however, in Figure 3. These membrane bones are: along the dorsal middle line, the parieto-frontals (p.f.), originally two pairs of bones which fuse in development, and the nasals (na.). Round the edge of the jaw, and bearing the teeth, are pre-maxillae (p.m.), and maxillae (mx.), and overlying the quadrate cartilage and lateral to the otic capsules are the T-shaped squamosal bones (sq.). In the ventral view of the skull (Figure 4) we see a pair of vomers (vo.) bearing teeth, a pair of palatines (pal.), [and a pair of pterygoids (pt.)] (which [palatines and pterygoids, we may note,] unlike those of the rabbit, are -stated to be- membrane bones), and a great median dagger-shaped para-sphenoid (p.sp.). These two Figures, and 5, which shows the same bones in side view, should be carefully mastered before the student proceeds with this chapter. The cartilage bones are distinguished from membrane bones by cross-shading.

Section 28. Turning now to Figure 8,I., we have a side view of a tadpole's skull. On the ventral side of the head is a series of vertical cartilaginous bars, the visceral arches supporting the walls of the tadpole's gill slits. The first of these is called the hyoid arch (c.h.), and the four following this, the first (br.1), second, third, and fourth (br.4), branchial arches. Altogether there are four gill slits and between the hyoid arch and the jaw arch, as it is called (= Meckel's cartilage + the palato-pterygoid), is "an imperforate slit," which becomes the ear-drum.* The frog no longer breathes by gills, but by lungs, and the gills are lost, the gill slits closed, and the branchial arches consequently much reduced. Figures 8, II., and 8, III., show stages in this reduction. The hyoid arch becomes attached, to the otic capsule, and its median ventral plate, including also the vestiges of the first, second, and fourth branchial arches, is called the hyoid apparatus. In Figure 5, the apparatus is seen from the side; c.h. is called the (right) anterior cornu** of the hyoid. The function of the hyoid apparatus in the frog is to furnish, a basis of attachment to the tongue muscles; it remains cartilaginous, with the exception of the relic of one branchial arch, which ossifies as the thyro-hyal (Figure 7 th.h.). It will be noted that, as development proceeds, the angle of the jaw swings backward, and the hyoid apparatus, shifts relatively forward. These changes of position are indicated in Figure 8, III., by little arrow-heads.

* We may note here that, comparing the ear of the frog with that of the rabbit, there is no external ear. There is, moreover, no bulla supporting the middle ear, and the tympanic membrane stretches between the squamosal in front and the anterior cornu of the hyoid behind. A rod-like columella auris replaces the chain of ear ossicles, and may, or may not, answer to the stapes alone, or even possibly to the entire series. In the internal ear there is no cochlea, and the otic mass is largely cartilaginous instead of entirely bony.

** Plural cornua.

Section 29. Before proceeding to the comparison of the mammalian skull with this, we would strongly recommend the student thoroughly to master this portion of the work, and in no way can he do this more thoroughly and quickly than by taking a parboiled frog, picking off the skin, muscle, and connective tissue from its skull, and making out the various bones with the help of our diagrams.

Section 30. Figure 9 represents, in the most diagrammatic way, the main changes in form of the essential constituents of the cranio-facial apparatus, as we pass from the amphibian to the mammalian skull. F. is the frog from the side and behind; b.c. is the brain-case, o.c. the otic capsule, e. the eye, n.c. the nasal capsule, p.p. the palato-pterygoid cartilage, mx. the maxillary membrane bones, sq. the squamosal, and mb. the mandible. The student should compare with Figure 5, and convince himself that he appreciates the diagrammatic rendering of these parts. Now all the distinctive differences in form, from this of the dog's skull (D.), are reducible to two primary causes--

(1) The brain is enormously larger, and the brain-case is vastly
inflated, so that--

(a) the otic capsule becomes embedded in the brain-case wall;

(b) the palato-pterygoid rod lies completely underneath the brain-case instead of laterally to it;

(c) the squamosal tilts down and in, instead of down and out, and the lower jaw articulates with its outer surface instead of below its inner, and, moreover, with the enormous distention of the brain-case it comes about that the squamosal is incorporated with its wall.

(2) The maxilla anteriorly and the palatine posteriorly send down palatine plates that grow in to form the bony palate, cutting off a nasal passage (n.p.) from the mouth cavity (m.p.), and carrying the posterior nares from the front part of the mouth, as they are in the frog, to the pharynx. Hence the vomers of the dog lie, not in the ceiling of the mouth, but in the floor of this nasal passage.

Section 31. The quadrate cartilage of the frog is superseded by the squamosal as the suspensorium of the lower jaw. It is greatly reduced, therefore; but it is not entirely absent. In the young mammal, a quadrate cartilage can be traced, connected with the palato-pterygoid cartilage, and articulating with Meckel's cartilage. Its position is, of course, beneath the squamosal, and just outside the otic capsule. As development proceeds, the increase in size of the quadrate, does not keep pace with that of the skull structures. It loses its connection with the palato-pterygoid, and apparently ossifies as a small ossicle-- the incus of the middle ear. A small nodule of cartilage, cut off from the proximal end of Meckel's cartilage, becomes the malleus. The stapes would appear to be derived from the hyoid arch. Hence these small bones seem to be the relics of the discarded jaw suspensorium of the frog utilized in a new function. Considerable doubt, however, attaches to this interpretation-- doubt that, if anything, is gaining ground.

Section 32. The tympanic bulla of the dog is not indicated in Diagram 9, and it would appear to be a new structure (neomorph), not represented in the frog.

* Faint vestigial indications occur in the developing skulls of some insectivora.

-Cranium_

-Nasal_ (paired), -Vomer_ (paired)
-Fronto-Parietal_, Sphenethmoid Bone (median), Eye, Pro-otic Bone,
Otic Cartilage, Ex-occipital (paired)
-Para-sphenoid Bone_

-Upper Jaw_

-Pre-Maxilla_ (paired), -Palatine_ (paired), Pterygoid (paired),
-Squamosal_, Quadrate Cartilage {To 1.}
-Maxilla_
1. Quadrato-Jugal

<-Lower Jaw_

Mento-meckelian, -Dentary_, -Articulare- [-Angulo Splenial_]

(1) The otic capsule (= periotic bone) of the dog ossifies from a number of centres, one of which is equivalent to the frog's prootic.

(2) The several constituents of the lower jaw are not to be distinguished in the adult mammal.

(3) The frog has no lachrymal bone.

Section 36. We are now in a position to notice, without any danger of misconception, what is called the segmental theory of the skull. Older anatomists, working from adult structure only, conceived the idea that the brain-case of the mammal represented three inflated vertebrae. The most anterior had the pre-sphenoid for its body, the orbito-sphenoids for its neural processes, and the arch was completed above by the frontals (frontal segment). Similarly, the basi-sphenoids, ali-sphenoids, and parietals formed a second arch (parietal segment), and the ex-, basi-, and supra-occipitals a third (occipital segment). If this were correct, in the frog, which is a more primitive rendering of the vertebrate plan, we should find the vertebral characters more distinct. But, as a matter of fact, as the student will perceive, frontal segment, parietal segment, and occipital segment, can no longer be traced; and the mode of origin from trabeculae and para-chordals show very clearly the falsity of this view. The vertebrate cranium is entirely different in nature from vertebrae. The origin of the parietals and frontals as paired bones in membrane reinforces this conclusion.

Section 37. But as certainly as we have no such metameric segmentation, as this older view implies, in the brain-case of the frog, so quite as certainly is metameric segmentation evident in its branchial arches. We have the four gill slits of the tadpole and their bars repeating one another; the hyoid bar in front of these is evidently of a similar nature; and that the ear drum is derived from an imperforate gill slit is enforced by the presence of an open slit (the spiracle) in the rays and dog-fish in an entirely equivalent position. Does the mouth answer to a further pair of gill slits, and is the jaw arch (palato-pterygoid + Meckel's cartilage) equivalent to the arches that come behind it? This question has been asked, and answered in the affirmative, by many morphologists, but not by any means by all. The cranial nerves have a curious similarity of arrangement with regard to the gill slits and the mouth; the fifth nerve forks over the mouth, the seventh forks over the ear drum, the ninth, in the tadpole and fish, forks over the first branchial slit, and the tenth is, as it were, a leash of nerves, each forking over one of the remaining gill slits. But this matter will be more intelligible when the student has worked over a fish type, and need not detain us any further now.

Section 38. See also Section 13 again, in which is the suggestion that the occipital part of the skull is possibly a fusion of vertebrae, a new view with much in its favour, and obviously an entirely different one from the old "segmental" view of the entire skull, discussed in Section 36.

Section 1. In the dog-fish we have a far more antique type of structure than in any of the forms we have hitherto considered. Forms closely related to it occur among the earliest remains of vertebrata that are to be found in the geological record. Since the immeasurably remote Silurian period, sharks and dog-fish have probably remained without any essential changes of condition, and consequently without any essential changes of structure, down to the present day. Then, as now, they dominated the seas. They probably branched off from the other vertebrata before bone had become abundant in the inner skeleton, which is consequently in their case cartilaginous, with occasional "calcification" and no distinct bones at all. Unlike the majority of fish, they possess no swimming bladder-- the precursor of the lungs; but in many other respects, notably in the uro-genital organs, they have, in common with the higher vertebrata, preserved features which may have been disguised or lost in the perfecting of such modern and specialized fish as, for instance, the cod, salmon, or herring.

Section 2. Comparing the general build, of a dog-fish with that of a rabbit, we notice the absence of a distinct neck, and the general conical form; the presence of a large tail, as considerable, at first, in diameter as the hind portion of the body, and of the first importance in progression, in which function the four paddle-shaped limbs, the lateral fins, simply co-operate with the median fin along the back for the purpose of steering; and, as a consequence of the size of the tail, we note also the ventral position of the apertures of the body. The anus, and urinary and genital ducts unite in one common chamber, the cloaca. Behind the head, and in front of the fore fin (pectoral fin), are five gill slits (g.s.) leading from the pharynx to the exterior. Just behind the eye is a smaller and more dorsal opening of the same kind, the spiracle (sp.). On the under side of the head, in front of the mouth, is the nasal aperture (olf.), the opening of the nasal sac, which, unlike the corresponding organ of the air-frequenting vertebrata, has no internal narial opening. There is, however, a groove running from olf. to the corner of the mouth, and this, closing, in the vertebrate types that live in air and are exposed to incessant evaporation of their lubricating secretions, constitutes the primitive nasal passage. The limbs are undifferentiated into upper, lower, and digital portions, and are simply jointed, flattened expansions.

Section 3. The skin of the dog-fish is closely set with pointed tooth-like scales, the placoid scales, and these are continued over the lips into the mouth as teeth. Each scale consists of a base of true bone, with a little tubercle of a harder substance, dentine, capped by a still denser covering, the enamel. The enamel is derived from the outer layer of the embryonic dog-fish, the epiblast, which also gives rise to the epidermis; while the dentine and bony base arise in the underlying mesoblast, the dermis. A mammalian tooth has essentially the same structure: an outer coat of enamel, derived from epiblast, overlies a mass of dentine, resting on bone, but the dentine is excavated internally, to form a pulpcavity containing blood-vessels and nerves. Most land animals, however, have teeth only in their mouths, and have lost altogether the external teeth which constitute the armour of the dog-fish. Besides the teeth there perhaps remain relics of the placoid scales in the anatomy of the higher vertebrata, in the membrane bones. How placoid scales may have given rise to these structures will be understood by considering such a bone as the vomer of the frog. This bone lies on the roof of the frog's mouth, and bears a number of denticles, and altogether there is a very strong resemblance in it to a number of placoid scales the bony bases of which have become confluent. In the salamander, behind the teeth-bearing vomers comes a similar toothed parasphenoid bone. The same bone occurs in a corresponding position in the frog, but without teeth. In some tailed amphibians the vomers and splenials are known to arise by the fusion of small denticles. These facts seem to point to stages in the fusion of placoid bases, and their withdrawal from the surface to become incorporated with the cranial apparatus as membrane bones, a process entirely completed in the mammalian type.

Section 4. The alimentary canal of the dog-fish, is a simple tube thrown into a Z shape. The mouth is rough with denticles, and has a fleshy immovable tongue on its floor. In the position of the Eustachian tube there is a passage, the spiracle (sp.), running out to the exterior just external to the cartilage containing the ear. The pharynx communicates with the exterior through five gill slits (g.s.), and has, of course, no glottis or other lung opening. There is a wide oesophagus passing into a U-shaped stomach (st.), having, like the rabbit's, the spleen (sp.) on its outer curvature. There is no coiling small intestine, but the short portion, receiving the bile duct (b.d.) and duct of the pancreas (pan.), is called the duodenum (d'dum.). The liver has large left (L.lv.) and right lobes, and a median lobe (M.lv.), in which the gall bladder (g.bl.) is embedded. The next segment of the intestine is fusiform, containing a spiral valve (Figure 4), the shelf of which points steeply forward; it is sometimes called the colon (co.). It is absorptive in function and probably represents morphologically, as it does physiologically, the greater portion of the small intestine. A rectal gland (r.g.) opens from the dorsal side into the final portion of the canal (rectum).

Section 5. The circulation presents, in many respects, an approximation to the state of affairs in the developing embryos of the higher types. The heart (Figure 3, Sheet -14- {Error in First Edition} [16]) is roughly, Z shape, and transmits only venous blood. It lies in a cavity, the pericardial cavity (P.c.c.), cut off by a partition from the general coelome. At one point this partition is imperfect, and the two spaces communicate through a pericardio-peritoneal canal (p.p.c.), which is also indicated by an arrow (p.p.) in the position and direction in which the student, when dissecting, should thrust his "seeker," in Figure 1 Sheet 15. A sinus venosus (s.v. in Figure 3, Sheet 16) receives the venous trunks, and carries the blood through a valve into the baggy and transversely extended -auricle- [atrium] (au.), whence it passes into the muscular ventricle (Vn.), and thence into the truncus arteriosus. This truncus consists of two parts: the first, the conus or pylangium (c.a.), muscular, contractile, and containing a series of valves; the second, the bulbus or synangium (b.a.), without valves and pulsatile. In the rabbit both sinus and truncus are absent, or merged in the adjacent parts of the heart.

Four efferent branchials (e.br.) carry the aerated blood on to the dorsal aorta (d.ao.). A carotid artery runs forward to the head, and a hypo-branchial artery supplies the ventral side of the pharyngeal region. There are sub-clavian, coeliac, mesenteric, and pelvic arteries, and the dorsal aorta is continued through the length of the tail as the caudal artery (Cd.A.).

Section 7. A caudal vein (Cd.V.), bringing blood back from the tail, splits behind the kidneys (K.), and forms the paired renal portal veins (r.p.v.), breaking up into a capillary system in the renal organ. A portal vein brings blood from the intestines to the liver.

Section 8. Instead of being tubular vessels, the chief veins of the dog-fish are, in many cases, irregular baggy sinuses. Three main venous trunks flow into the sinus venosus. In the median line from behind comes the hepatic sinus (H.S.); and laterally, from a dorsal direction, the Cuvierian sinuses (C.S.) enter it. These, as the student will presently perceive, are the equivalents of the rabbit's superior cavae. They receive, near their confluence with the sinus venosus, the inferior jugular vein (I.J.V.). At their dorsal origin, they are formed by the meeting of the anterior (A.C.S.) and posterior (P.C.S.) cardinal sinuses. The anterior cardinal sinus -is, roughly, the equivalent of the internal jugular vein-, lies along dorsal to the gill slits (g.s.), and receives an orbital sinus from the eye. The posterior cardinal sinus receives a sub-clavian vein (s.c.v.) and a lateral vein (L.V.), and fuses posteriorly with its fellow in the middle line. This median fusion is a departure from the normal fish type. It must not be confused with the inferior cava, which is not found in the dog-fish, the [right] posterior cardinals representing the rabbit's azygos vein. A simplified diagram of the circulation of a fish is given in Figure 2, Sheet 16, and this should be carefully compared with the corresponding small figure given of the vascular system of our other types.

{Lines from Second Edition only.}
[The blood of the dog-fish resembles that of the frog.]

Section 9. The internal skeleton, as we have said, is entirely cartilaginous, and only those parts which are pre-formed in cartilage in the skeletons of the higher types are represented here. The spinal column consists of two types of vertebrae, the trunk, bearing short, distinct, horizontally-projecting ribs (r.), and the caudal. The diagrams of Figure 5 [(Sheet 18)] are to illustrate the structure of the centrum of a dog-fish vertebra; C is a side view, D a horizontal median section, A and B are transverse sections at the points indicated by -B and A- [A and B] respectively in Figure C. -(By an unfortunate slip of the pen in the figure, A was substituted for B; section A corresponds to line B, and vice versa.)- The vertebrae are hollowed out both anteriorly and posteriorly (amphi-coelous), and a jelly-like notochord runs through the entire length of the vertebral column, being constricted at the centres of the centra, and dilated between them. The neural arch above the centrum, and containing the spinal cord, is made up of neural plates (n.p.), and interneural plates (i.n.p.), completed above by a median neural spine (n.s.). In the caudal region, instead of ribs projecting outwardly, there are haemal processes, inclined downwards and meeting below, forming an arch, the haemal arch, containing the caudal artery and vein-- the vein ventral to the artery-- and resembling the neural arch, which contains the spinal cord above, in shape and size.

Section 10. The pectoral limb and girdle (Figure 4, Sheet 16) have only a very vague resemblance to the corresponding structures in the rabbit. The girdle (g.) is a transverse bar lying ventral to the pericardial wall, and sending up a portion (sc.), dorsal to the attachment of the limb, which answers to the scapula and supra-scapula of the forms above the fish. Three main cartilages, named respectively the pro- (p.p.), meso- (m.p.), and meta-pterygium, form the base of the limb. With these, smaller cartilaginous plates, rods, and nodules articulate, and form a flattened skeletal support for the fin.

Section 11. The pelvic girdle and limb (Figure 2, Sheet 15) are similar in structure, but the pro-pterygium and meso-pterygium are absent, and the cartilage answering to the meta-pterygium goes by the name of the basi-pterygium. In the male, but not in the female, the pelvic fins are united behind the cloaca, and there are two stiff grooved copulatory organs, the claspers (cl. in Figure 1), which have a cartilaginous support (cl.c.). These claspers form the readiest means of determining the sex of a specimen before dissection.

Section 12. The skull consists of a cartilaginous cranium, and of jaw and visceral arches. The cranium persists throughout life, in what closely resembles a transitory embryonic condition of the higher types. There is a nasal capsule (na.c.), a brain case proper, and lateral otic (auditory) capsules (ot.c.) containing the internal ear. (This should be compared with the frog's embryonic skull.) The upper jaw has a great bar of cartilage, the palato-pterygoid, as its sole support; the arch of premaxilla, maxilla, jugal, and squamosal-- all membrane bones-- is, of course, not represented. In the frog this bar of cartilage is joined directly to the otic capsule by a quadrate portion, but this is only doubtfully represented in the dog-fish by a nodule of cartilage in the pre-spiracular ligament (p.s.). The lower jaw is supported, by Meckel's cartilage (M.C.). The hyoid arch consists of two main masses of cartilage, the hyomandibular (h.m.), and the ceratohyal (c.h.); the former of these is tilted slightly forward, so that the gill slit between it and the jaw arch is obliterated below, and the cartilage comes to serve as the intermediary in the suspension of the jaw from the otic mass. There are five branchia[l] arches, made up pharyngo-, epi- and cerato-branchials, and the ventral elements fuse in the middle line to form a common plate of cartilage. Outside these arches are certain small cartilages, the extra branchials (ex.b.) which, together with certain small labials by the nostrils and at the sides of the gape, probably represent structures of considerably greater importance in that still more primitive fish, the lamprey. The deep groove figured lateral to the otic capsule is the connecting line of the orbital and anterior cardinal sinuses; the outline of the anterior cardinal sinus in this figure and in Figure 1 is roughly indicated by a dotted line.

Section 13. Figure 3a is a rough diagram of the internal ear-- the only auditory structure of our type (compare Rabbit, Sheet 7). To dissect out the auditory labyrinth without injury is a difficult performance, but its structure may be made out very satisfactorily by paring away successive slices of the otic mass. Such a section is shown by Figure 3b; through the translucent hyaline cartilage the utriculus and horizontal canal can be darkly seen. The ductus endolymphaticus (vide Rabbit) is indicated by a dotted line in our figure. It is situated internal to the right-angle between the two vertical canals, and reaches to the surface of the otic capsule.

Section 14. The brain shows the three primary vesicles much more distinctly than do our higher types. The fore-brain has large laterally separated olfactory lobes (rh.), there are relatively small "hemispheres" (pr.c.), the stalk of the pineal gland tilts forward, and the gland itself is much nearer the surface, being embedded in the cartilage of the brain case, and the pituitary body is relatively very large, and has lateral vascular lobes on either side. Following the usual interpretation of the parts, we find optic lobes (op.l.) as the roof of the mid-brain, and behind a very large, median, hollow, tongue-shaped cerebellum (c.b.). The medulla is large, and certain lateral restiform tracts (r.t.) therein, which also occur in the higher types, are here exceptionally conspicuous.

The fifth nerve, has, in the dog-fish, as in the rabbit, three chief branches. V.2 and V.3 fork over the mouth just as they do in the mammal; V.1 passes out of the cranium by a separate and more dorsal opening, and runs along a groove along the dorsal internal wall of the orbit, immediately beneath a similar branch of VII., which is not distinct in the rabbit. The grooves are shown in the figure of the cranium, Sheet 18; the joint nerve thus compounded of V. and VII. is called the ophthalmic (oph.). It is distributed to the skin above the nose and orbit. When the student commences to dissect the head of a dog-fish he notices over the dorsal surface of the snout an exudation of a yellowish jelly-like substance, and on removing the tough skin over this region and over the centre of the skull he finds, lying beneath it, a quantity of coiling simple tubuli full of such yellowish matter. These tubuli open on the surface by small pores, and the nerves terminate in hair-like extremities in their lining. These sense tubes are peculiar to aquatic forms; allied structures are found over the head and along a lateral line (see below) in the tadpole, but when the frog emerges from the water they are lost. They, doubtless, indicate some unknown sense entirely beyond our experience, and either only possible or only necessary when the animal is submerged.

In addition to the ophthalmic moiety mentioned above, the seventh nerve has a vidian branch (vid.) running over the roof of the mouth, and besides this its main branches fork over the spiracle, just as V. forks over the mouth, and as IX. and X. fork over gill clefts. This nerve in the rabbit is evidently considerably modified from this more primitive condition.

The eighth is the auditory nerve, as in the rabbit.

The ninth nerve forks over the first branchial cleft.

The tenth nerve is easily exposed by cutting down through the body wall muscles over the gill clefts, into the anterior cardinal sinus (A.C.S.). It gives off (a) branches forking over the posterior four gill slits, (b) a great lateral nerve running inward, and back through the body-wall muscle, and connected with a line of sense organs similar to those in the head, the lateral line, and (c) a visceral nerve curving round to the oesophagus and stomach. In dissection it becomes very evident that the tenth nerve is really a leash of nerves, each one equivalent to the ninth.

We may here call the attention of the reader to the fact of the singular resemblance of V., VII., IX., and the factors of X. That each has a ventral fork, we have already noticed. Each also (?IX.) has a dorsal constituent connected with the sense organs of the skin. The vidian branch of VII., however, is not evidently represented in the others.

Section 16. The coelom of the dog-fish is peculiar-- among the types we treat of-- in the possession of two direct communications with the exterior, in addition to the customary indirect way through the oviduct. These are the abdominal pores (a.p.) on either side of the cloaca in either sex. They can always be readily demonstrated by probing out from the body cavity, in the direction indicated by the arrow (a.p.) in Figure 1, Sheet 15. They probably serve to equalize the internal and external pressure of the fish as it changes its depth in the water, just as the Eustachian tubes equalize the pressure on either side of the mammal's tympanic membrane.

Section 17. The musculature of the dog-fish body is cut into V-shaped segments, the point of the V being directed forward. The segments alternate with the vertebrae, and are called myomeres. Such a segmentation is evident, though less marked, in the body wall muscles of the frog, and in the abdominal musculature of the rabbit and other mammals it is still to be traced.

Section 18. The uro-genital organs of the female dog-fish (Figure 1, Sheet 17) consist of an unpaired ovary (ov.), paired oviducts (o.d.), enlarged at one point to form an oviducal gland (o.d.g.), kidneys (k.), with ureters (ur.) uniting to form a urinary sinus (u.s.) opening into the cloaca by a median urinary papilla separate from the oviducal openings. The eggs contain much yolk, and, like those of the fowl, are very large; like the fowl, too, one of the ovaries is suppressed, and it is the right ovary that alone remains. The two oviducts meet in front of the liver ventral to the oesophagus, and have there a common opening by which the ova are received after being shed into the body cavity. The eggs receive an oblong horny case in the oviduct; in the figure such a case is figured as distending the duct at e. The testes of the male (T. in Figure 2) are partially confluent in the middle line. They communicate through vasa efferentia (v.e.) with the modified anterior part of the kidney, the epididymis (ep.), from which the vas deferens (v.) runs to the median uro-genital sinus (u.g.s.), into which the ureters (ur.) also open. The silvery peritoneum (lining of the body cavity) covers over the reddish kidneys, and hides them in dissection.

Section 19. Figure 3, Sheet 17, is a generalized diagram of the uro-genital organs in the vertebrata; M.L. is the middle line of the body, G. is the genital organ, Pr. is the pronephros, or fore kidney, a structure which is never developed in the dog-fish, but which has functional importance in the tadpole and cod, and appears as a transitory rudiment in the chick. A duct, which is often spoken of as the pronephric duct (p.d.), and which we have figured under that name, is always developed. Anteriorly it opens into the body cavity. It is also called the Mullerian duct, and in the great majority of vertebrata it becomes the oviduct, uniting with its fellow, in the case of the dog-fish, ventral to the oesophagus. In the male it usually disappears; the uterus masculinus of the rabbit is still very generally regarded as a vestige of it. Kolliker has shown, however, that this interpretation is improbable. Ms. is the mesonephros, some or all of which becomes the epididymis in the male of types possessing that organ, and is connected with G. by the vasa efferentia. Mt., the metanephros, is, in -actual fact- [the frog], indistinguishably continuous with Ms., and is the functional kidney, its duct (metanephric duct) being either undifferentiated from the mesonephric (as is the case with the frog) or largely split off from it, as in the dog-fish, to form the ureter.

(a) the testes, vasa efferentia, and epididymis of each side to shift posteriorly until they reach a position on either side of the cloaca; and

(b) The uro-genital apertures, instead of meeting dorsally and posteriorly to the anus, to shift round that opening and meet anteriorly and ventrically to it.