The anatomy of the frog

4. The fourth spinal nerve (N. spinalis IV) (Fig. 126 M4) appears between the fourth and fifth vertebrae; it runs on the ventral surface of the transverse process of the fifth vertebra and upon the Musc. intertransversarius obliquely outwards and downwards, and reaches the deeper surface of the Musc. obliquus internus, into which it descends, about opposite the articulation between the eighth and ninth vertebrae; it then bifurcates into:‍—

(1) A Ramus cutaneus abdominalis, which pierces the muscle and supplies the skin.

(2) A Ramus muscularis, which supplies the broad abdominal muscle and the M. rectus abdominis.

5. The fifth spinal nerve (N. spinalis V) (Fig. 126 M5) emerges through the foramen between the fifth and sixth vertebrae, runs obliquely downwards and outwards over the transverse processes of the sixth and seventh vertebrae and the corresponding M. intertransversarii, and at a point nearly opposite the middle of the urostyle pierces the M. obliquus internus.

The rest of its course corresponds exactly with that of the fourth spinal nerve.

6. The sixth spinal nerve (N. spinalis VI) (Fig. 126 M6), after escaping from the vertebral canal between the sixth and seventh vertebrae, runs obliquely downwards and outwards on the under surfaces of the transverse processes of the seventh, eighth, and ninth vertebrae, then under the iliac bone to a point opposite the hinder half of the urostyle, where it descends under cover of the M. obliquus abdom. internus. In the rest of its course it repeats the corresponding courses of the fourth and fifth spinal nerves.

7, 8, 9, and 10. The seventh, eighth, ninth, and tenth spinal nerves (Nervi spinales VII, VIII, IX, and N. spinalis X s. N. coccygeus) (Figs. 126 and 127 M7, M8, M9, and M10) are best described together, as they are intimately associated to form the sciatic plexus (Plexus ischiadicus, Plexus cruralis).

Within the vertebral canal the roots of these nerves form the Cauda equina; the seventh nerve leaves the canal between the seventh and eighth vertebrae, the eighth nerve between the eighth and ninth vertebrae, the ninth between the sacrum or ninth vertebra and the urostyle, and the tenth by the Canalis coccygeus in the urostyle; from these points the four nerves run obliquely on the ventral surface of the M. ilio-coccygeus to the pelvis, where they form the sciatic plexus. Although subject to some variation, the usual arrangement is that the seventh and eighth nerves unite to form a trunk, which then receives the ninth nerve; the large nerve so formed is the sciatic nerve: a branch of the tenth usually then joins either the sciatic plexus or the sciatic nerve; the plexus gives off or receives the following branches:‍—

(a) Like other spinal nerves, these nerves give off Rami communicantes (see Sympathetic System).

(b) The M. ilio-hypogastricus (Fig. 127 M7a). This arises from the seventh spinal nerve, before its union with the eighth spinal nerve; it runs obliquely outwards and downwards on the Musc. ilio-coccygeus and under the border of the M. obliquus abdom. internus, where it bifurcates:‍—

(1) One branch, the Ramus cutaneus abdominalis, pierces the M. obliq. abdom. internus in the same fashion as the cutaneous branches of the fourth, fifth, and sixth spinal nerves; it supplies the skin of the abdomen.

(2) The second or Ramus muscularis supplies the flat abdominal muscles and the M. rectus abdominis.

(c) The M. cruralis (Fig. 127 M7b) also arises from the seventh spinal nerve at or just beyond its point of union with the eighth spinal nerve. It accompanies the A. cruralis on the pelvic wall to the thigh, where it lies upon the M. ilio-psoas in a triangle between the M. rectus fem. anterior and the M. adductor longus, and divides into two main branches:‍—

(1) Muscular twigs to the M. ilio-psoas, M. rectus femoris anticus, MM. adductores longus and brevis.

(2) The Ramus cutaneus femoris, which runs downwards in the hinder wall of the Lymph-sac (Saccus iliacus), and supplies the skin of the under and outer surfaces of the thigh.

(d) The Ramus dorsalis is a very small nerve; according to Waldeyer it possesses only twenty nerve-fibres. It arises from the coccygeal nerve immediately beyond the Canalis coccygeus, perforates the M. ilio-coccygeus, and runs on its dorsal surface obliquely over the lymph-heart, without supplying it; the nerve then pierces the fascia to supply the skin of the back and thigh.

(e) The Ramus abdominalis also arises from the coccygeal nerve; it is larger than the R. dorsalis, and arises at the same point; the nerve runs on the ventral surface of the M. ilio-coccygeus towards the lymph-heart, and inosculates with the sympathetic. It is from this nerve that the branch to the sciatic plexus is usually given off; it further supplies a varying number of branches, which with branches derived directly or indirectly from the sciatic nerve, form a plexus (Plexus coccygeus); the two plexuses may together be regarded as a Plexus ischio-coccygeus or a Plexus sacro-coccygeus.

(f) In addition to the Rami communicantes given off by the spinal nerves near the vertebral column, the sciatic plexus supplies a few twigs, usually two (Fig. 127).

(g) Other small twigs pass from the sympathetic system forwards and outwards to join the sciatic plexus or the sciatic nerve. According to Waldeyer two of these are very constant.

(h) Branches to the oviduct (Fig. 127 b).

(i) Branches to the rectum (Fig. 127 cc).

(k) Branches to the bladder (Fig. 127 d).

(l) Branches to the M. levator ani (Fig. 127 a′).

(m) A branch to the lymph-heart, which runs along the anterior border of the M. lev. ani, on to its dorsal surface, and then direct to the lymph-heart.

Variations in the sciatic plexus. The arrangement of the nerves in the sciatic plexus is subject to many variations; according to Wiedersheim most of these variations belong to two chief classes: either the N. coccygeus inosculates directly with the ninth spinal nerve by one or several branches, or it joins the ninth nerve indirectly by uniting with its branches. A case of the latter arrangement is seen in Fig. 127, and is thus described by Wiedersheim:‍—

‘After cutting through the pelvic symphysis and drawing to one side the contained viscera, namely, the hinder end of the oviduct, the rectum, and the bladder, one sees a row of small twigs (a, b, cc, d, and e) arising from the inner, hinder, and anterior surfaces of the ninth spinal nerve: the first (a) runs backwards parallel with the N. coccygeus over the M. ilio-coccygeus to join this nerve at the point marked Com. The twig cc behaves in like manner after receiving a twig from b. The twig b arises from the inner surface of the ninth nerve between the two foregoing and close to its union with the eighth spinal nerve; from this origin twig b passes almost transversely outwards to the hinder extremity of the oviduct (“uterus”), and partly to the rectum, crossing in its course twig a, the sciatic nerve, and the urostyle. Its branches form a net-like plexus with the terminal branches of the last sympathetic ganglion and with a branch cc from the point of union Com.

‘A second branch (a′), arising from the point of junction Com, is a continuation of the N. coccygeus (M10); it passes vertically downwards towards the hinder end of the cloaca and at the upper border of the Levator ani, divides into two branches, which are distributed to the inner and outer surfaces of this muscle and to the cloaca. Other branches pass dorsally to the lymph-hearts, while a third set pass to the hindermost part of the bladder.

‘These three sets of nerves, to the M. coccygeus, M. levator ani, and to the lymph-hearts, are not supplied entirely by the N. coccygeus, as this is reinforced by one or more branches (e) from the sciatic plexus: this branch (e) supplies twigs to the M. ilio-coccygeus, which is also supplied anteriorly from the trunk of the N. coccygeus.

‘The bladder receives a special branch (d), which arises from the sciatic plexus at the junction of the eighth and ninth spinal nerves; this nerve gives a twig (d′) to the M. iliacus.’

I. The sciatic nerve (N. ischiadicus) (Fig. 128 I) is the largest nerve of the body; it passes under the M. coccygeus, between the origins of the M. vastus externus and of the M. pyramidalis: lies then between the M. biceps and the M. pyramidalis, and later between the M. biceps and the M. semimembranosus; lastly, it bifurcates under the M. biceps to form the N. tibialis (II) and the N. peroneus (III). In this course it gives off:‍—

(a) A twig to the M. coccygeo-iliacus, while still in the pelvis.

(b) The N. cutaneus femoris posterior (Fig. 128, 2), which passes between the M. pyramidalis and the M. vastus externus, to appear behind and beneath the former; it accompanies an artery of like name to supply the skin of the hinder and inner surfaces of the thigh.

(c) A little below the foregoing it gives off a collateral branch (Fig. 128, 3), which passes under the M. pyramidalis and divides to form‍—

(1) A branch to the upper third of the M. semimembranosus.

(2) A branch to the Rectus internus minor. A twig of this branch (bb) pierces the muscle transversely in company with the Arteria cutanea, and passes to the skin of the middle of the inner surface of the thigh.

(3) Branches to both heads of the semitendinosus.

(4) Branches to the M. adductor magnus.

(d) Branches covered by the M. pyramidalis to the M. quadrat. femoris and M. obturator.

(e) A branch (5) forwards to the M. biceps.

(f) A branch (6) which accompanies the Art. circumflexa genu lateralis sup. forwards to the M. extensores cruris, the M. vastus externus, and the M. rectus anterior.

II. The Nervus tibialis (Figs. 128 II, 129 T) passes backwards and inwards to supply the extensors of the foot and the flexors of the toes. Its branches are:‍—

(a) The R. cutaneus cruris posterior (Figs. 128 and 129 ct), accompanied by an artery and vein, passes to the skin of the calf.

(b) A twig to the upper part of the gastrocnemius (Fig. 129 g′). The main nerve, after supplying this twig, bifurcates to form the next two nerves.

(c) The Nervus suralis (Figs. 128 8; 129 s) runs down­wards on the inner border of the M. gastrocnemius and gives off:‍—

(1) A twig (g″) to the upper third of the muscle.

(2) The Ramus cutaneus medius (cs) is given off below the middle of the M. gastrocnemius; it passes to the skin of the lower third of the leg. After giving off the latter branch, the N. suralis runs downwards along the inner aspect of the Tendo achillis to the Aponeurosis plantaris of the sole of the foot as far as the calcar, and gives off:‍—

(3) Branches (flp) to the Musc. plantaris and the M. flexor digitorum.

(4) A branch to the M. abductor hallucis.

(5) The R. digitalis volaris I (dvI) supplies the inner side of the first toe by its terminal twig.

(d) The Nervus tibialis (Fig. 129 t) passes downwards on the hinder surface of the tibio-fibula, sinks into the M. tibialis posticus to appear again at its hinder border; it then runs over the ankle-joint into the sole of the foot, where it lies midway in the space between the two Ossa tarsi and between the M. extensor tarsi and the M. abductor digiti I longus; the nerve then runs downwards in the groove of the small Os tarsi and divides to form three branches:‍—

(1) The first branch (dvII) runs to the space between the second and third fingers, where it bifurcates:‍—

(α) The Rami digital. volaris I run transversely over the muscles of the second toe, supply the muscles of the first toe, and bifurcate to form the R. digital. volaris I and II.

(β) The Ram. digit. volar. II divides in the space between the second and third fingers to supply the Flex. phalang. and the adjacent sides of these toes.

(2) The second branch (dvIII) runs over the M. flex. metatarsi of the third toe to the space between the third and fourth toes, and bifurcates to supply the adjacent sides of these toes and the web between them.

(3) The third branch at once divides to supply:‍—

(α) The M. transv. metatarsi.

(β) The MM. lumbricales of the fourth toe.

(γ) The M. abductor digit. V (ab5), the M. flexor brevis dig. V (F5), and the M. adductor dig. V (ad5); it then ends as‍—

(δ) A bifurcating branch forming the R. digit. volar. of the fourth and fifth toes.

III. The Nervus peroneus (N. peroneus communis superior, Ecker), (Fig. 130 pc) is the second division of the sciatic nerve; it passes between the outer head of origin of the M. gastrocnemius and the tendon of insertion of the M. biceps, it then lies on the tibio-fibula between the M. gastrocnemius and the M. peroneus, where it is accompanied by the Vena tibialis postica, then runs downwards upon the M. extensor cruris and the Flexor tarsi ant., and passes under the M. tibial. anticus and bifurcates; its branches are:‍—

a. The N. cutaneus cruris lateralis (Fig. 128 cp); which, like other cutaneous nerves, runs in a common sheath together with an artery and vein to the skin. It arises close to the sciatic nerve.

b. Muscular branches to the MM. peroneus, tibialis anticus, extensor cruris brevis, and the flexor tarsi anterior.

c. The N. peroneus medialis (Fig. 130 pm) is the smaller of the two terminal branches of the N. peroneus; it courses with the Art. tibialis antica under the MM. flexores tarsi anterior and posterior and supplies the latter.

d. The N. peroneus lateralis (Fig. 130 pl) is the larger terminal branch of the N. peroneus; it passes downwards between the heads of the M. tibialis anticus to the tendon of origin of the Flexor tarsi posterior, where it gives off two branches (Nos. 1 and 2); the nerve then unites with the N. peroneus lateralis to form a common stem, the N. peroneus communis inferior (Ecker). The N. peroneus lateralis gives off:‍—

(1) The N. cutaneus dorsi pedis lateralis (cpl), which passes to the skin of the outer side of the dorsum of the foot.

(2) A second branch to the M. extensor of the fourth and fifth toes.

e. The N. peroneus communis inferior (Fig. 130 pci) runs on the dorsum of the foot in company with the Art. dorsalis pedis, underneath the MM. extensores digiti I and II; it supplies several branches and then bifurcates. It gives off:‍—

(1) Branches to the MM. extensor longus and brevis digiti I.

(2) Rami digitales dorsales to the adjacent sides of the first and second toes.

(3) Branches to the MM. extensores digiti II.

(4) The inner, terminal branch (Fig. 130 A) at once divides:‍—

(α) The outer branch runs between the third and fourth toes as far as the commencement of the web, where it bifurcates to form two Rami cutanei, which course along the adjacent sides of these toes as far as their apices.

(β) The inner branch passes to the extensor muscle, and, in part, to the adjacent sides of the third and fourth toes.

(5) The outer, terminal branch (Fig. 130 B) of the N. peroneus communis inferior passes to the muscles of the fourth and fifth toes, and supplies Rami cutanei dorsales to the outer side of the fourth and inner side of the fifth toes.

Cutaneous branches of the N. tibialis supply the outer side of the fifth and inner side of the first toe.

G. The Sympathetic System (Sympathicus).

(Re-written by the translator.)

The sympathetic cord or chain is a row of nervous ganglia (vertebral or lateral ganglia), connected by nerve-fibres, and lying on either side of the vertebral column (Figs. 117 and 131); with the exception of the last spinal nerve there is usually one sympathetic ganglion associated with each spinal nerve; in the case of the tenth spinal nerve there may be only one ganglion or as many as twelve.

The first ganglion (Figs. 111, opposite ics; 117 S1) is placed on the hypoglossal nerve just as it emerges from the first intervertebral foramen; it is large, but smaller than the second; its Ramus communicans is represented by several fine and very short fibres, which connect the ganglion with the nerve. This ganglion is connected with the second by two or three nervous threads, between which passes the subclavian artery, a true Annulus Vieussenii being thus formed (de Watteville). The other ganglia are connected by single bands of fibres. The first ganglion supplies also branches to the axillary artery and to the cardiac plexus.

The second ganglion (Figs. 117 and 131) is the largest, and is closely applied to the brachial nerve; as in the case of the first ganglion and hypoglossal nerve, it is attached to the second spinal nerve without a distinctly marked Ramus communicans.

The third ganglion (Ganglion cardiacum basale, Gaskell and Gadow) is sometimes fused with the second, but is usually close to the third spinal nerve: it has a short but distinct Ramus communicans.

Behind the third ganglion the sympathetic cord is continued backwards along the corresponding aortic arch, then parallel with and close to the abdominal aorta (Figs. 117, 127, and 131), receiving Rami com­mu­ni­can­tes, which are long and well marked, from each of the spinal nerves; the fourth, fifth, and sixth nerves usually supply each one Ramus com­mu­ni­cans, the seventh two, and the eighth and ninth each two or three: from the tenth nerve it receives three or more, as many as twelve having been noted. The ganglia are usually more or less spindle-shaped or flattened and triangular; the hinder part of the cord usually receives in addition two or three branches from the sciatic plexus and twigs from the R. abdominalis of the N. coccygeus.

The branches and communications of the sympathetic cords are as follows:‍—

a. Communicating branches between the two cords; these are extremely numerous and irregular, forming a net-like plexus, which surrounds the abdominal aorta and other adjacent structures, and gives off numerous small twigs to the neighbouring vessels and organs.

b. Communications with the cranial nerves; these are two branches (Wiedersheim), (Figs. 111 and 116) which pass from the first ganglion to the Ganglion nervi vagi, where one terminates, the other leaves the ganglion to pass on to the Gasserian ganglion (Figs. 111 and 116 VS): according to Gaskell a single nerve passes from the first ganglion to the Ganglion nervi vagi, whence one portion of its fibres is continued to the Gasserian ganglion, the remainder accompanying the pneumogastric nerve without any connection with the ganglion; he therefore names this nerve the vago-sympathetic. (See Gasserian ganglion, p. 168.)

c. Communications with the spinal nerves; these are:‍—

(1) The Rami communicantes.

(2) Communications between the sympathetic ganglia or their branches and the spinal nerves or their branches (Fig. 117); by means of these connections, fibres of the sympathetic system are conducted by the spinal nerves and their branches to all parts of the body.

d. Branches to the heart, which form the following ganglia:‍—

(1) A relatively large plexus lies on the auricles in the median plane immediately beneath the division of the M. hyoglossus. It supplies a network of fibres to the auricles and the adjacent large vessels. It is said to communicate at various points with the pneumogastric nerve.

(2) A smaller ganglion of oval form, supplies twigs to the neighbouring vessels and a communicating branch to the hypoglossal nerve (Wiedersheim).

e. Branches to the abdominal viscera; these form intricate plexuses by which the organs are supplied: the one best known is the solar plexus (Fig. 131); it is formed chiefly from branches derived from the third, fourth, and fifth ganglia, and supplies the stomach, etc.; other plexuses for the various viscera are known by corresponding names, such are the Plexus hepaticus, renalis, genitalis, haemorrhoidalis, and vesicalis.

The sympathetic system is characterised by the fact that the branches form intricate plexuses, which include numerous ganglia and which are very irregular; it is also characteristic that most of its fibres are non-medullated. According to the investigations of Gaskell, the fibres of the sympathetic system arise in mammalia as very fine medullated fibres from the posterior vesicular (Clarke’s) columns (Mason has recently described cells in the frog’s spinal cord, which he holds to be homologous with the cells of these columns); they leave the cord by both the ventral and dorsal roots of the spinal nerves, and are thus connected with two sets of ganglia, (1) with the ganglia of the dorsal roots, and (2) through the Rami communicantes with the sympathetic ganglia (vertebral or lateral ganglia); these two sets of ganglia Gaskell terms proximal. By means of the branches from the sympathetic ganglia (Rami efferentes) part of the fibres pass to another set of ganglia, the solar plexus, etc., which he terms prevertebral or collateral; from these the fibres pass to be distributed to the various viscera and blood-vessels, where a fourth set of very small ganglia (terminal ganglia) is found. The prevertebral and terminal ganglia are together classed as distal ganglia. Gaskell holds that the fine medullated fibres from the cord lose their medullary sheath in one or other of these ganglia according to the function they fulfil.

The inhibitory fibres of the heart and vaso-dilator fibres of the blood-vessels continue as white fibres along the vago-sympathetic and spinal nerves to the distal ganglia (Bidder’s ganglion, etc.), where the medullary sheath disappears: whereas the ‘augmentor’ fibres of the heart and vaso-constrictor fibres of the blood-vessels lose their medullary sheath in the proximal ganglia and pass on as non-medullated fibres. In the same way the nerve-fibres that bring about contraction of the circular muscle fibres of the hollow viscera lose their medullary sheaths in the proximal ganglia, while those fibres, the influence of which negatives the former, become pale fibres in the distal ganglia.

Waters has demonstrated that in the frog the various spinal nerves have each a localised physiological action upon the blood-vessels and muscular walls of various parts of the alimentary canal: he shows that

The third spinal nerve supplies the oesophagus.

The fourth spinal nerve supplies the stomach.

The fifth spinal nerve supplies the upper third of the small intestine.

The sixth spinal nerve supplies the lower two thirds of the small intestine.

The seventh spinal nerve supplies the large intestine.

The eighth spinal nerve supplies the bladder, this supply being, however, not so definite as the others given above.

It has long been known that the branches of the spinal ganglia (ganglia of the posterior roots) are together larger (one-third, Lenhossék), and contain more fibres than the ventral and dorsal roots together; this is supposed to be in part due to an acquisition of new fibres derived from the ganglia. The majority of recent observers hold that each ganglionic cell has only one process, which, however, soon bifurcates; whether any of the fibres so formed pass as far as the cord or beyond its blood-vessels is doubted by most observers, and denied by Gaskell. These remarks and the description of the cells of the spinal ganglia (p. 176) hold good for the lateral or vertebral ganglia and the prevertebral ganglia (solar ganglion, etc.) of the sympathetic system; the terminal ganglia will be described with the organs in which they are found.

H. Histological Notes on the Nervous System.

(In order to render the foregoing description of the nervous system more complete, the following notes have been added by the translator.)

[The histological elements of the nervous system are nerve-cells and nerve-fibres; of these the nerve-cells have been described with the parts in which they occur; it may simply be remarked that later observers have been unable to discover the ‘spiral cells’ described by Beale, Arnold, and others. The nerve-fibres, as in most other animals, are of two kinds, medullated and non-medullated.

1. Medullated nerve-fibres or white fibres are found in all cranial and spinal nerves, with the exception of the olfactory nerves, and in many of the sympathetic nerves (see Sympathetic System); also in the white matter of the brain and spinal cord; examined microscopically the fibres are seen to consist of an external sheath or neurilemma, a medullary sheath, and an axis-cylinder:‍—

a. The neurilemma (Sheath of Schwann, Outer or Primitive Sheath) is a nucleated endothelial layer covering the nerve-fibre; it is continuous with the corresponding coat of the nerve-cells, and is uninterrupted throughout the length of the nerve; at the nodes, however, it dips down towards the axis-cylinder, the circular groove so formed being filled with cement substance.

b. The medullary sheath (White substance of Schwann); the presence of this sheath is the chief cause of the whiteness of these nerves; the thickness of the sheath varies considerably, and towards the ultimate distribution of the nerve it is entirely lost. At more or less regular intervals along the course of the nerve-fibre the continuity of the medullary sheath is broken, and gives the fibres the appearance of being constricted at these places; such constrictions are known as nodes of Ranvier; the portion between two such nodes being termed an internode. Each internode possesses an oval, flattened, granular nucleus at about its middle and placed between the neurilemma and the medullary sheath; the nucleus has a nucleolus. In the fresh state the medullary sheath seems to be fluid; it is of a fatty nature.

Medullary segments are caused by breaks in the continuity of the medullary sheath, which are seen only in nerves which are no longer in their normal condition, and are especially well marked after treatment with osmic acid. The breaks are oblique; hence the conical end of one segment fits into the funnel-shaped end of the next. How far they correspond to pre-existent structures is uncertain. The segments in the frog vary in length from 0.010 to 0.040 mm.

c. The axis-cylinder is the essential part of every nerve-fibre; it shows a longitudinal striation corresponding to the fine fibrils (primitive fibrillae) of which it consists; these fibrils often exhibit minute varicosities: at times it has the appearance of being invested with a very delicate structureless sheath. It is continuous through the nodes of Ranvier.

2. Non-medullated nerve-fibres (Grey or Varicose Fibres; Fibres of Remak); these occur chiefly in branches and plexuses of the sympathetic system; they consist of a neurilemma and an axis-cylinder, which agree exactly with the corresponding elements found in the medullated fibres.

The nerve-fibres, whether medullated or non-medullated, are bound together by connective-tissue to form nerves. A number of fibres bound together by connective-tissue to form a slender cord is known as a funiculus; a small nerve may consist of one such funiculus; the sheath surrounding it is known as the perineurium, and sends in supporting processes between the fibres (endoneurium): when several funiculi are bound together to form a large nerve the common sheath is known as the epineurium. These sheaths support nerves (nervi nervorum) and vessels (vasa vasorum) supplying the nervous elements, and their intercellular spaces form lymph-canals. The whole nerve is surrounded by an endothelial coat, which helps to form a lymph-space, which more or less completely surrounds the nerve.

The ultimate distribution of the nerve-fibres will be included in the description of the various organs in which they end.]