The vertebral column of the frog consists of ten bones, viz. nine true vertebrae, and the rod-shaped urostyle, which alone is almost as long as all the remaining vertebrae.

I. General Description of the nine Vertebrae.

a. The bodies of the vertebrae are compressed from above downwards; the posterior surface of each body, with the exception of the eighth, presents an articular head covered with cartilage; the anterior surfaces, with the exception of the ninth, present corresponding articular depressions, covered with cartilage.

b. The arches, which have somewhat sharp margins both before and behind, bear the following processes:— 1. The articular processes (Figs. 4 and 5 o o) are similarly placed to those of the dorsal vertebrae of man: they project horizontally, the cartilaginous articular surfaces on the posterior processes being directed downwards, those on the anterior upwards.

2. The transverse processes (Figs. 4 and 5 t t′) are strong, flat, and of very varying size and direction. The transverse processes of the fourth vertebra are the longest, those of the third only a little shorter; the shortest are those of the seventh and eighth. The atlas has no transverse processes. Those of the second and third vertebrae project directly outwards and slightly downwards; those of the fourth, fifth, and sixth upwards and backwards. The seventh and eighth project more directly outwards and at the same time backwards; the ninth upwards and markedly backwards. All the transverse processes have cartilaginous epiphyses; the largest are those of the second, third, fourth, and ninth vertebrae.

3. The spinous processes are gen­er­al­ly small, but in­di­vidu­ally of varying size, appearance, and direction. The longest are those of the third, fourth, and fifth vertebrae; these are, in transverse section, of a three-sided prismatic form, as in the dorsal vertebrae of man; they are directed backwards and provided with car­til­agin­ous epiphyses. The spinous processes of the sixth and seventh are shorter, compressed from side to side, project directly upwards, and are usually without cartilaginous epiphyses; that of the eighth is still shorter. As regards the spinous processes, those of the third, fourth, and fifth vertebrae resemble those of the dorsal vertebrae in man; those of the sixth, seventh, and eighth, lumbar vertebrae. The ninth has either no spinous process or only a rudimentary one. The first and second vertebrae may be looked upon as cervical vertebrae: the second has a short spinous process with a cartilaginous epiphysis. In the first, the cartilage which unites the two halves of the arch represents the rudiment of a spinous process.

II. Description of Particular Vertebrae.

1. The atlas or first vertebra has a thin body, compressed from above downwards, and an arch. The body has posteriorly a slightly raised, cartilaginous, articular head, which is broader transversely: in front it has two oval articular facets, which are separated from each other by a median projection. Each facet is concave, and directed forwards, outwards, and slightly upwards. The arch is completed above by cartilage, which projects slightly to form the rudiment of a spinous process. The hinder margin of the arch bears two articular processes. Transverse processes are wanting.

2. The second vertebra presents all the general characters of an ordinary vertebra, except that the transverse processes are directed somewhat downwards.

3. The transverse processes of the third vertebra are longer than those of the second: each is directed downwards, is broader at its extremity than at its base, and bears a hammer-shaped cartilaginous epiphysis larger than those of the remaining transverse processes.

4. The transverse processes of the fourth vertebra are the longest: each is broader at its free end than at its base, is directed upwards and backwards, and provided with a cartilaginous epiphysis.

5, 6, 7. The transverse processes of the fifth, sixth, and seventh vertebrae are smaller, contracted towards their free extremities, and directed upwards.

8. The eighth vertebra is distinguished from the rest by its body possessing no articular head. It presents, at each end, a concave articular depression. The transverse processes resemble those of the seventh.

9. The ninth vertebra unites the vertebral column with the hip-bones, and is hence to be regarded as a sacrum. The body bears on its anterior surface an articular head for articulation with the eighth vertebra: on its posterior surface are two small rounded and closely approximated processes (Figs. 4 and 5 sc) for articulation with the urostyle. The transverse processes are strong, broader at the free ends than at their origin, directed upwards and backwards, and provided with cartilaginous epiphyses.

III. Articulations of the Vertebrae.

The articular heads and depressions of the vertebral bodies, together with the joint surfaces of the articular processes, are covered with hyaline cartilage. The periosteum of the bodies, as also that of the articular processes, forms true capsular joint ligaments. The articulations of the vertebrae are still further strengthened by longitudinal fibres, which extend along the anterior and posterior surfaces of the vertebrae, and correspond to the ligamentum vertebrale commune anticum et posticum of man. Between the vertebral arches are membranes which represent the ligamenta intercruralia. Between the spinous processes are bands of connective tissue which form ligamenta interspinalia. (For the articulation of the atlas with the occiput, see page 24.)

IV. Structure of the Vertebrae‍28 (Figs. 6 and 7).

Each vertebral body consists of a cylinder of compact bone, which is directly continued into the bony substance of the arch. In the interior of the cylinder is found an isolated persistent vestige of the chorda dorsalis (Ch): this is surrounded by cancellous bone (c), which extends backwards towards the articular head and forwards directly into the articular cartilage, compact bone being absent in these parts. In a transverse section of a vertebral body the following parts are seen (Fig. 6):—a. An outer layer of compact bone (o) (the transverse section of the above-mentioned cylinder), which is formed of parallel lamellae of varying thickness. These, according to Gegenbaur, and as I can confirm, are arranged in well-defined groups, each of five to eight lamellae. The number of the secondary lamellae increases with the age of the animal. b. In the interior, in the form of a cylinder, is the remnant of the chorda dorsalis. It consists of a double sheath (Ch′) and contents (Ch) composed of chorda-cells. c. Immediately around the persistent portion of the chorda lies the central part of the vertebral body, formed by transformation of the vertebral cartilage and of the bases of the original cartilaginous arches. At each side of the chorda are large marrow-spaces (c), filled with cells, from which proceed narrower canals, winding in various directions, and anastomosing freely with one another both before and behind. Their walls are constituted partly of true bone, partly of cartilage.

V. The Urostyle (Figs. 8 and 9).

The urostyle is a long, median, rod-like bone, which projects backwards, midway between the two hip-bones, and terminates over the anus. The anterior end (Fig. 8 a) is the thicker and broader part of the bone, and has two articular depressions (Fig. 9) for articulation with the two facets of the ninth vertebra. The hinder end is pointed and cylindrical, and terminates in a cartilage, which is fixed in the tubular end of the bone. The middle portion is almost cylindrical, and has a groove along the ventral surface which gradually becomes less marked behind. The dorsal surface bears a ridge (Figs. 8 and 9 s), which is high and thick in front, becomes sharper and less prominent as it proceeds backwards, and gradually disappears towards the hinder third of the bone, so that in transverse section the anterior two-thirds of the bone appear triangular, with a ventral and two lateral surfaces: while the hinder third is cylindrical. The anterior portion of the bone contains a canal, canalis vertebralis (Fig. 9 c.v.), which is a continuation of the vertebral canal, along which the hindermost spinal nerves pass. On each side of the anterior portion of the urostyle are small apertures (Fig. 8 c.c.), which lead into canals (canales coccygei), which open into the vertebral canal, and through which the coccygeal nerves pass. In front of these openings and partly overhanging them are small triangular projections (Fig. 8) (processus transversarii): these, however, are not constant, and are more often found in R. esculenta than in R. temporaria, in which latter species the openings are smaller.

II. THE SKULL.

The flat form of the frog’s head, as in batrachians generally, depends upon the wide separation of the jaw-bones of the two sides, and on the large size of the orbital cavities and the horizontal direction of their floors. The outer circumference of the head forms a parabolic frame (Figs. 10 and 11), composed of the maxillary (m), premaxillary (i), and quadratojugal bones (j). In the middle of this curved framework lies the elongated prismatic cranium. Anteriorly, this is attached to the fore-part of the frame by means of the cartilaginous skeleton of the organs of smell (Fig. 11 e′); posteriorly, it widens out into two transverse arms (p), which contain the organs of hearing. From this base, on either side, a bony strut, composed of the posterior arms of the squamosal (t′) and of the pterygoid bones, passes backwards to the hinder end of the frame. The anterior arm of the squamosal bone (t) does not quite reach the framework, but is attached to it by ligament alone. Between the last-named arm posteriorly, the cranium on the inner side, and the maxillary frame-work laterally, is a large space representing the orbital and temporal fossae of human anatomy.

A. The Cranium.

The cranium of the frog is a prismatic tube, wide behind, narrow in front, and formed in great part of cartilage (Figs. 15 and 17). Our indigenous species are characteristically distinguished from one another by peculiarities in the form of the cranium. In R. esculenta it is long and narrow, in R. temporaria short and wide. The superior surface in the former is markedly concave, while in the latter it is flat, and in R. oxyrhinus arched. These differences are readily recognised in the living animal.

The Bones of the Cranium.

1. The exoccipital bones, ossa occipitalia lateralia, Cuvier (Figs. 10, 11, 12, 14, 16 o).

These paired bones form the hinder part of the cranium; they bound the foramen magnum, and articulate with the vertebral column. They are imbedded in the cartilaginous matrix of the skull, and are separated above by an unossified part of this matrix (occipitale superius, Dugès), which represents the tabular portion of human anatomy: below they are separated by a similar part (occipitale basilare, Dugès) which represents the body of the occipital bone. They therefore properly represent only the condylar portions (partes condyloideae) of the human occipital bone. Each possesses a cartilaginous articular head, for articulation with the first vertebra: these converge below, and surround the lower half of the circumference of the foramen magnum. This latter has, in R. esculenta, a transversely oval outline; in R. temporaria, a somewhat heart-shaped outline, with the apex directed upwards: in accordance with this the whole bone is wider than high in the first species; and in the latter it is higher than it is wide. From the upper and outer border of the foramen magnum on each side a ridge runs obliquely outwards and downwards, in which lies the suture between this bone and the prootic bones. This bony ridge (processus mastoideus, autt.) is usually cartilaginous in R. esculenta, even in old animals; in R. temporaria, even in young specimens, it is bony. In the latter species the bones unite very early, while in the former they remain separated by the primitive cartilage. Between this crest and the processus condyloideus there is a depression (fossa condyloidea), with a hole (foramen condyloideum) through which the vagus nerve leaves the cranium. The exoccipital take part in the formation of the labyrinth of the ear, as will be noticed later on.

Articulation of the Exoccipital Bones with the Atlas. From the middle of the anterior surface of the body of the atlas a ligament arises, representing to a certain extent the lig. suspensorium dentis, and attached to the basal portions of the exoccipital bones.

2. The prootic bones, ossa petrosa, Cuvier (Figs. 10, 11, and 12 p).

These paired bones lie at the sides and in front of the exoccipital bones. As already explained, they remain in R. esculenta separated from these by cartilage, while in R. temporaria they early enter into bony union with them; this is due to the complete ossification of the processus mastoideus in the latter species, as stated above. The prootics form the lateral expansions of the posterior part of the skull in which the organs of hearing are placed. The large cavity which contains the ear labyrinth is completed by the exoccipital: internally it opens freely into the skull, and externally on the posterior wall of the skull through the foramen ovale, which is formed by both these bones. The postero-lateral part of the prootic usually remains cartilaginous: at the side and in front of the foramen ovale this cartilage is pierced by a small opening, through which passes the nervus facialis or ramus tympanicus n. vagi (Volkmann). At the side there is a process to which the suspensorium of the lower jaw is attached (Fig. 12 p): behind this is a hollow in which the auditory ossicles lie, and which may be designated fossa tympanica (Fig. 12 t). The anterior border of the bone forms the hinder and inner walls of the orbit. Here also is the trigeminal foramen (Fig. 11 p″) through which the N. trigeminus and the several nerves for the muscles of the eye pass; it represents the foramen ovale, for. rotundum, and the fissura orbitalis superior (sphenoidal fissure) of the human sphenoid bone. The foramen is sometimes, especially in young animals, only a notch, which is completed by cartilage. On account of the relation of this part (Fig. 11 p′) of the bone to the nerves which pierce it, the whole bone has been named by Stannius the ala magna or temporalis of the sphenoid; it has been also looked upon as a bone which contains these elements, as by Dugès, who on this account calls it rupéo-ptéréal.

Appendages of the Prootic Bones.

1. The styloid cartilage. From the cartilaginous portion of the prootic the styloid cartilage runs downwards, backwards, and inwards, and is continued directly into the anterior cornu of the hyoid bone (Figs. 11 h′ and 12 h).

2. The auditory ossicles.

3. The tympanic ring (annulus tympanicus) is an annular cartilaginous frame; or more exactly, has the shape of a short, truncated cone, as it narrows towards the middle line: it is attached to the squamosal bone. (See Organ of hearing.)

3. The parasphenoid, os sphenoideum, Cuvier (Figs. 11 and 16 s).

A large portion of the base of the cranium is taken up by this cruciform bone. Of the two longitudinal median processes, the posterior is by far the shorter, and lies in front of and partly below the cartilaginous os occipitale basilare. The anterior longer longitudinal arm closes in the greater part of the cranium from below, and articulates by its outer edges with that part of the prootic bones often described as the alae magnae, and also with the cartilage lying in front, which forms the greater part of the lateral walls of the cranium. The anterior extremity of the bone articulates with the palatine bones. The transverse arms lie on the under surface of the exoccipitals and of the prootics.

The greater width of the cranium in R. temporaria is associated with the greater relative width of the anterior arm of this bone.

4. The fronto-parietal bones, ossa fronto-parietalia, Cuvier (Figs. 10 and 14 fp).

These are a pair of somewhat long, flat bones, which form the principal part of the upper wall or roof of the cranium, and cover in superiorly the cartilaginous cranium, which is here, in great part, persistent. They are united in the middle line by the sagittal suture; posteriorly they articulate with the exoccipital and prootic bones; anteriorly with the sphenethmoid, which they overlap like tiles. The outer margin of each bone is bent somewhat downwards (Fig. 16 fp), and between it and the parasphenoid there is a space in the wall of the cranium which is closed in by cartilage and connective tissue only.

These bones are narrower in R. esculenta, and along the sagittal suture are depressed into a groove: where the superior surface bends down to become lateral the edges are much more prominent. In R. temporaria the bones are broader and flat or even somewhat arched. The latter condition is still more marked in R. oxyrhinus.

5. The sphenethmoid, os ethmoideum (Figs. 10, 11, 14, and 16 e).

The long tubular cranium is completed anteriorly by a single bone, which forms at once the roof, floor, and lateral walls. It is consequently more or less ring-shaped, on which account it has been named ‘os en ceinture’ by Cuvier. Only the posterior portion is annular, however: the anterior portion forms a double canal, with a median partition, for the passage of the nerves of smell, and as these canals are widened out anteriorly, this part of the bone helps to complete the nasal cavities, which, however, are bounded for the most part by cartilage, as described below. In some species of frogs (as for example R. occellata, Rathke) this cartilage is partly ossified.

The sphenethmoid has on each side a small bony canal, running forwards and inwards, through which the ramus nasalis of the first division of the trigeminal nerve passes.

The cartilaginous skeleton of the nose (Figs. 14 and 16 n, n″, n‴, n⁗). The anterior borders of the funnel-shaped cavities of the sphenethmoid pass into cartilage, which forms two capsules, separated from each other by a median cartilaginous septum, and opening laterally. We can distinguish, (a) a cartilaginous septum, forming a continuation of the bony one; (b) the floor of the nasal cavity, narrower behind, wider in front; (c) a roof somewhat narrower than the floor. The floor and roof are united in front by an arched surface. From this cartilaginous capsule various processes project, which unite it to other portions of the facial skeleton: firstly, from the most posterior portion of the capsule there passes transversely outwards a bar of cartilage (sn), which, widening, becomes continuous with the cartilage (sp″) forming the basis of the anterior arm of the pterygoid bone. From the anterior angle a cartilaginous process passes outwards (Figs. 14, 16 n″), which is attached to a projection on the anterior end of the maxillary bone; from the same spot a horn-shaped cartilaginous process (n″, n‴) curves round backwards and towards the middle line. This projection bounds the nasal cavity externally, sending off a free process (n‴) on the way, and ends with a double point on the roof of the cartilaginous nasal cavity (n⁗). On the floor of the nasal capsule, on either side, is a knob-shaped cartilaginous eminence, running from behind, forwards and outwards; this may be regarded as an indication of a turbinated bone. From the anterior wall there passes into each nasal cavity an almost horizontal, partly ossified plate (c.n.), ending posteriorly in a free pointed border. These are the cornets of Dugès (l. c., p. 12, Pl. I, Figs. 1, 5), who correctly regarded them as turbinated bones. Cuvier described them as rudimentary nasal bones‍29.

The alar car­til­ages of the nose have still to be de­scribed. They are (a.n.) shell-shaped car­til­ages, hol­lowed out on the inner sides and de­crea­sing in width from before back­wards. They are movably attached by their anterior broader ends to the tips of the projecting portions of the premaxillary bones, and are so arranged as to bound the lateral margins of the nasal apertures, which they overlap.

6. The cartilaginous basis of the skull, Primordial cranium (Figs. 14, 16).

a. If the fronto-parietals be removed, (Figs. 14, 16), a cartilage (s′) is found which partially closes the cranial cavity, and which is usually perforated on either side by a foramen (f′). More anteriorly in the median line is found a space (f) which extends to the sphenethmoid bone, and is only closed by connective tissue: posteriorly the cartilage extends between the exoccipitals as far as the foramen magnum, representing the os occipitale superius.

b. The base of the cranium is (Fig. 16) also partly cartilaginous. If the parasphenoid (s) be removed we find above it a cartilage (s′) which passes backwards between the exoccipitals, and extends as far back as the foramen magnum: it corresponds to the os occipitale basilare.

c. The sides of the cranium are formed almost entirely of cartilage. This cartilage fills the space bounded by the nasals above, the parasphenoid below, the so-called ala magna of the prootic bone behind, and the sphenethmoid in front; and has in its posterior part an aperture (r) through which passes the nervus opticus, and below this a smaller one (r′) for the nervus abducens. The extent of the cartilage in the antero-posterior direction is greater in R. esculenta than in R. temporaria, or in other words the sphenethmoid stretches further back in the latter than in the former.

B. The Bones of the Face.

The Suspensorium.

7. The squamosal bones, ossa tympanica, Cuvier (Figs. 10, 14 t).