Chapter XIV: Section III: The Nervous System (4)

‘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_.

Distribution of the sciatic nerve.

_I_ The sciatic nerve.
_II_ N. tibialis.
_III_ N. peroneus.
1 Branches to the M. pyramidalis.
2 N. cut. fem. posterior.
3 Large collateral branch.
5 Branch to the M. biceps.
6 Branch accompanying the art. circumfl. genu lateralis sup.
7 Branch of tibial nerve to the M. gastrocnemius.
8 N. suralis.
_ad_‴ M. adductor magnus.
_b_ M. biceps.
_bb_ Twig accompanying the art. cutanea.
_ci_ M. coccygeo-iliacus.
_cp_ N. cut. cruris lateralis.
_ct_ R. cut. cruris posterior.
_g_ M. gastrocnemius.
_gl_ M. glutaeus.
_p_ M. pyramidalis.
_pe_ M. peroneus.
_ra_ M. rectus anterior.
_ri_′ M. rectus internus minor.
_sm_ M. semimembranosus.
_st_ M. semitendinosus.
_ve_ M. vastus externus.
]

*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.

Nerves of the leg and sole of the foot.

_ab5_ Branch to the M. abduct. digit. V.
_ad5_ Branch to the M. adduct. digit. V.
_ar_ M. rectus anterior.
_b_ M. biceps.
_cs_ Ramus cutaneus medius.
_ct_ Ramus cutaneus cruris posterior.
_dvI_ Ramus digitalis volaris I.
_dvII_ Branch supplying R. digit. volaris I and II.
_dvIII_ Second branch of the N. tibialis.
_F5_ Branch to the M. flex. brev. digit. V.
_flp_ Branches to the M. plantaris and the M. flexor digitorum.
_g_′ Branch to the M. gastrocnemius.
_g_″ Branch to the M. gastrocnemius.
_pc_ N. peroneus.
_s_ N. suralis.
_sm_ M. semimembranosus.
_T_, _t_ Nervus tibialis.
]

(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_.

Distribution of the _N. peroneus_.

_A_ Inner terminal twig of N. peron. comm. inf.
_B_ Outer terminal twig of N. peron. comm. inf.
_cpl_ N. cutaneus dorsi pedis lateralis.
_pc_ N. peroneus.
_pci_ N. peroneus communis inferior.
_pl_ N. peroneus lateralis.
_pm_ N. peroneus medialis.
]

*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 _S_1) 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_.

Sympathetic cord. From Ecker (Icones physiologicae, Pl. XXIV, Fig. 3).

The heart, lungs, and liver have been removed; the stomach, intestine, kidneys, and testes drawn to the right side; the left sympathetic cord is thus pulled to the right side to expose the Rami communicantes.

_S_ Sympathetic cord attached to the ganglion of the vagus.

The numbers refer to the ganglia, which are enumerated from before backwards. ]

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.]