Chapter XIII: Section III: The Nervous System (3)

_a_ Branch of the Ram. palatinus to the Harderian gland.
_b_ Branch of the Ram. palatinus to the intermaxillary gland.
_c_ Inosculating-branch of the Ram. palatinus.
_Ch_ Internal naris.
_d_ Cutaneous branch of the Ram. palatinus.
_db_ Fibrous plate.
_F_ Ramus mandibularis of the facial nerve.
_Gl_ Intermaxillary gland.
_HD_ Harderian gland.
_l_ Eyeball.
_m_ M. masseter.
_Ms.ob.i._ M. obliquus inferior.
_OK_ Upper jaw.
_r_ M. retractor bulbi.
_ri_ M. rectus internus.
_ri_′ M. rectus inferior.
_sc_ M. sterno-cleido-mastoideus.
_uk_ Mandible.
† Terminal branch of Ram. palatinus.
* Union of facial nerve with Ram. anterior of the
glossopharyngeal.
_III_ Motor oculi nerve.
_Vb_ Ramus palatinus of trigeminal nerve.
_Vc_ Ramus maxillo-mandibularis of the trigeminal nerve.
_Vc_′ Ramus maxillaris of the trigeminal nerve.
_Vd_ R. mandibularis.
_Ve(VII)_ Facial nerve.
_X_ Pneumogastric nerve.
_X1_ Glossopharyngeal nerve.

Fig. 114[57]. Lateral dissection of head, etc., to show the cranial
nerves, etc.

[Footnote 57: The branches of the vagus which are represented in this figure as crossing the petrohyoid muscles ought to be underneath them.--_Trans._]

_c_ Cardiac nerve.
_F_ Union of facial and Ram. anterior of the
glossopharyngeal nerve.
_h_ Ram. hyoideus of the glossopharyngeal nerve.
_la_ M. levator angi scapulae.
_lgd_ M. longissimus dorsi.
_m_ M. masseter.
_mt_ Ram. mandibularis of the glossopharyngeal nerve.
_M1_ Hypoglossal nerve.
_M2_ Brachial nerve.
_Oh_ Anterior cornu of the hyoid bone.
_oi_ M. obliquus internus.
_ph1_ M. petrohyoideus I.
_ph2_ M. petrohyoideus II.
_ph3_ M. petrohyoideus III.
_ph4_ M. petrohyoideus IV.
_pp_ Pharyngeal branches of the pneumogastric nerve.
_tt_′ M. temporalis.
_us_ Muscular twigs of the Ram. mandibularis of the
trigeminal nerve.
_Z_ Tongue.
_Va_ Terminal twigs of the ophthalmic nerve.
_Vc_ Ram. maxillaris of the trigeminal nerve.
_Vd_ Ram. mandibularis of the trigeminal nerve.
_Ve(VII)_ Facial nerve.
_X1_ Ram. anterior of the glossopharyngeal nerve.
_X2_ Glossopharyngeal nerve.
_X3_ Pneumogastric nerve.
_X3g_ Ram. gastrici of the pneumogastric nerve.
_X3l_ Ram. laryngeus.
_X3p_ Ram. pulmonales of the pneumogastric nerve.
_XI_ Accessory nerve of the pneumogastric nerve.

Fig. 115. Dissection of the floor of the mouth.

_dm_ M. depressor maxillae.
_F_ United facial nerve and Ram. anterior of the
glossopharyngeal nerve.
_gh_ } M. geniohyoideus.
_gh_′ }
_h_ Ram. hyoideus of the glossopharyngeal nerve.
_H_ Heart.
_Lg_ Lung.
_m_′ Ram. mandibularis of the glossopharyngeal nerve.
_M1_ Hypoglossal nerve.
_M2_ Brachial nerve.
_Oh_ Anterior cornu of the hyoid bone.
_oh_ Momohyoideus.
_ph1_ M. petrohyoideus I.
_ph2_ M. petrohyoideus II.
_ph3_ M. petrohyoideus III.
_ph4_ M. petrohyoideus IV.
_sc_ M. sterno-cleido-mastoideus.
_sh_ M. sterno-hyoideus.
_sm_ M. mylohyoideus (submaxillaris).
_smt_ M. submentalis.
_TT_ Thyroid.
_uk_ Mandible.
_us_ Muscular twigs of the Ram. mandibularis.
_X2_ Glossopharyngeal nerve.
_X3_ Pneumogastric nerve.
_X3c_ Rami cardiaci of pneumogastric nerve.
_X3l_ Ram. laryngeus of the pneumogastric nerve.
_X3p_ Ram. pulmonalis of the pneumogastric nerve.

II. THE PERIPHERAL NERVOUS SYSTEM.

E. THE CRANIAL NERVES.

(To facilitate reference the original arrangement of this part has been altered.)

[There are ten pairs of cranial nerves in the frog, which are numbered in order from before backwards. The mode of origin of these nerves, and their deeper relations with the respective parts of the brain to which they belong, have already been described.]

*1.* The *olfactory nerve* (_N. olfactorius_), [(Figs. 97, 98, 102, 103, 111 _I_) runs a very short course only a few lines in length, and escapes from the cranium by an opening in the cartilage of the sphenethmoid into the nasal cavity, where it divides into two branches, each of which breaks up into a brush of filaments, to be distributed in the olfactory mucous membrane. The nerve contains no white fibres]. (See organ of smell, p. 385.)

*2.* The *optic nerve* (_N. opticus_), (Figs. 97, 102, 103, 111 _II_) [arises, as already described, by the optic tract, and joins with its fellow at the optic chiasma or commissure, where part of the fibres pass over to the opposite side (according to Michel all the fibres cross). Each optic nerve then courses outwards, piercing the cartilage of the cranium and so reaching the eyeball]. (See organ of sight, p. 408.)

*3.* The *motor oculi* (_N. oculomotorius_, _Oculo-motor_, _Motor communis_), (Figs. 102, 111, 112, 113 _III_). From its origin it courses outwards and forwards, perforating with a slight obliquity the cartilaginous wall of the cranium, just in front of the Gasserian ganglion (Fig. 116 _III_); on reaching the orbit it divides into two branches, between the _Levator bulbi_ and the _Rectus internus_ and _inferior_. One branch, _Ramus superior_, runs over and parallel to the _Ramos ophthalmicus Trig._ and enters the under surface of the _Rectus superior_; the second, lower branch, _Ramus inferior_, supplies filaments to the _Rectus internus_ and _inferior_ and to the _Obliquus inferior_. Apparently it exchanges fibres with the ophthalmic division of the trigeminal nerve.

[That portion of the motor-oculi between its branch to the _Rectus superior_ and to that to the _Rectus internus_ contains a number of nerve cells; according to Schwalbe (_l. c._, p. 235 and Pt. XII, Fig. 4) these cells are arranged in four groups or ganglia; the second group forms a very slight prominence, and is covered by a very fine layer of nerve fibres, it also gives off some extremely fine nerves to the eyeball; this is perhaps a ciliary ganglion (_Ganglion ciliare_, Schwalbe). Beard, however, names it ‘mesocephalic ganglion.’ The other three groups are simply clusters of cells between the fibres of the nerve.]

*4.* The *pathetic* or *trochlear nerve* (_N. trochlearis_, _patheticus_), (Figs. 102, 103, and 112 _IV_). From its origin it courses forwards with and then crosses the motor-oculi; it perforates very obliquely the cartilaginous lateral wall of the cranium, in front of the motor oculi but above and very slightly behind the optic foramen (Fig. 116); it runs parallel to and in company with the _Ramus opthalmicus trigemini_, and appears to exchange a few fibres with it, which, however, according to de Watteville, Stannius, Cuvier, and Wyman, is not really the case. In this course it lies first to the inner, then to the outer side of the ophthalmic, over which it passes to supply the _Rectus superior_. [The pathetic and ophthalmic nerves are sometimes enclosed in a common sheath (de Watteville).]

*5.* The *trigeminal nerve* (_N. trigeminus_), (Figs. 97, 102, 103, 111, 112, 113, and 114 _V_) is the largest of the cranial nerves in the frog; from its origin it runs outwards and forwards to the skull wall, and just before reaching this enters the large _Gasserian ganglion_. It then passes through the cranial wall immediately in front of the auditory capsule, and divides at once into two main branches (Fig. 111 _Vg_, etc.), the _Ramus ophthalmicus_ and the _Ramus maxillo-mandibularis_.

The Gasserian ganglion is a large, yellow, oval ganglion placed in a depression in the outer wall of the cranium; it is covered by a ‘periganglionic gland,’ which is similar in structure to that of a spinal ‘periganglionic’ gland (see p. 180), the fibrous capsule being stronger. This ganglion receives three other nerves besides the trigeminal, _viz._ the sixth and seventh nerves, and branches of the sympathetic.

According to de Watteville, these nerves are arranged on the lower surface of the ganglion, so that the sympathetic is below, the sixth above it, then the seventh, while the fifth is uppermost; the sympathetic splits into several bundles which join the various branches of the ganglion; the sixth divides into two bundles, one of which usually joins the ophthalmic, while the other makes its exit independently; the seventh splits into two bundles, one of which leaves the ganglion as the hyomandibular branch, the other gives some fibres to the palatine branch of the trigeminal nerve, and is then continued as the palatine branch of the facial.]

*a.* The _Ramus ophthalmicus_ (_Ramus nasalis_, Fischer; Ophthalmic or Orbital Branch, Wyman; Orbito-nasal or Ophthalmic Nerve, de Watteville), (Figs. 111 and 112 _Va_). After leaving the Gasserian ganglion the nerve is directed forwards parallel to the side of the cranium, between this and the eyeball. It lies beneath the _Rectus superior_, but above all the other muscles of the eyeball and the optic nerve. At the anterior end of the orbit it divides into two branches (_e_ and _f_) which pass through apertures (_Foramen pro ramo nasali_) in the cartilage of the sphenethmoid, to reach the nasal cavity, where they lie between the cartilage and mucous membrane. In this position the nerves supply branches to the mucous membrane and then pierce the skull to appear on the surface (Figs. 111, 112 _g,g,g_, 114 _Va_), the skin of which they supply. The branches inosculate with each other and communicate with the anterior twigs of the _Ramus maxillaris_ (Fig. 114 _Vc_) and with those of the _Ramus palatinus_ (Fig. 113 _Vb_).

In its course through the orbit the _Ramus ophthalmicus_ gives off:--

(1) Near the Gasserian ganglion one, two, or three small branches, which communicate with the trochlear nerve, and may then be traced to the sclerotic of the eyeball; some of the twigs enter near the optic nerve, others nearer the cornea. They are regarded as _Ciliary nerves_ (_Nervi ciliares_).

(2) A large branch, the *palatine nerve* (_Ramus palatinus_), (Figs. 111 and 113 _Vb_, _b_). This nerve, after receiving the palatine branch of the facial, descends on the inner border of the _Levator bulbi_ to the mucous membrane of the mouth, to which it gives numerous filaments; it then runs parallel and near to the median line, lying on the base of the skull; and near the front of the orbit it divides into three branches. The smallest (_a_) supplies the Harderian gland (_HD_); the second (_b_) continues in the course of the original nerve, pierces the vomer, and divides, giving filaments to the mucous membrane in its whole course, while the terminal twigs supply the intermaxillary gland (_Gl_) and the surrounding structures. One of these (†) ascends on the anterior border of the sphenethmoid to the intermaxillary (internasal, Born) space to supply the numerous glands and to inosculate by one or two twigs with the nasal branch (_tr_) of the trigeminal. The third branch of the palatine nerve (_c_) curves directly outwards, just behind the palatine bone, to the inner surface of the maxillary bone; it then courses backwards, pierces the fibrous plate (_db_) between the eyeball and the pterygoid bone, and ultimately unites with superior maxillary division of the trigeminal nerve (_Vc_′). Just before piercing the fibrous plate it gives off a tolerably large nerve (_d_) to the mucous membrane, and in its whole course it gives off twigs to the mucous membrane and surrounding structures; many of these twigs inosculate with other terminal twigs of the trigeminal.

[This nerve has been minutely described by Stirling and Macdonald; these observers find that the branches form a very fine and close plexus in the mucous membrane, etc.; fibres were traced to blood-vessels, glands, etc., and ganglion cells were found scattered along the nerves. The fibres are both medullated and non-medullated, and many of the cells are described as ‘spiral cells’ (see page 201) such as those described by Arnold in the sympathetic system of the frog.]

*b.* The _Ramus maxillo-mandibularis_ (_Ramus maxillaris_, Ecker and Hoffmann; Supra-maxillary, Humphry; Upper Maxillary Branch, Wyman), (Figs. 111, 112, and 113 _Vc_). This nerve is the largest division of the trigeminal; it runs directly outwards behind the eyeball, in front of the auditory capsule, and between the temporal and pterygoid muscles. After a very short course it divides into the maxillary and mandibular nerves. The nerve gives off before its division a few fine filaments (_i,i_) to the hinder half of the upper eyelid and to the surrounding skin, also a branch (_h_) [not correctly drawn in Fig. 111] which divides to supply twigs to the temporal and pterygoid muscles.

(1) The _Ramus maxillaris_ (_Ramus supramaxillaris_, Ecker; _Ramus maxillaris superior_, Hoffmann; Upper Maxillary Branch, Wyman; Supramaxillary, Humphry and de Watteville), (Figs. 111, 112, 113, and 114 _Vc_) runs outwards and then forwards, between the eyeball and the outer wall of the orbit, to the margin of the upper jaw (_K_); a large portion terminates in a number of small branches for the supply of the skin of the lower eyelid, the upper lip, and of the parts between the tympanic membrane and the external nares. Some of these twigs inosculate with twigs of the palatine and ophthalmic nerves. The second portion of the nerve unites with the palatine nerve, as already described.

(2) The _Ramus mandibularis_ (_Ramus maxillaris inferior_, Hoffmann; Mandibular or Lower Jaw Branch, Wyman; Inframaxillary, Humphry and de Watteville), (Figs. 111, 112, 113, 114, and 115 _Vd_, _us_). As far as the outer margin of the eyeball, this nerve runs parallel to and behind the _Ramus maxillaris_, in which course it supplies branches to the temporal and pterygoid muscles; it then curves backwards, outwards, and downwards to the under surface of the squamosal bone, where it supplies a twig to the _Depressor maxillae_, and then perforates the _Masseter_: in this manner it reaches the outer surface of the mandible, just behind the insertion of the temporal muscle; it then courses forwards, under the skin, to the _Symphysis menti_. In this course the nerve gives off numerous branches to the skin and surrounding parts, it also supplies the mylo-hyoid and submental muscles; one branch (Figs. 114 and 115 _us_) is larger than the rest, and supplies the under surface of the floor of the mouth and lower lip.

*6.* The *abducens nerve* (_Nervus abducens_, Ecker and Hoffmann; included in the trigeminal, Wyman), (Figs. 111 and 112 _VI_). This very slender nerve courses along the inner wall of the cranium from its origin to the Gasserian ganglion (_q.v._) which it joins, and leaves this in contact with the ophthalmic division of the trigeminal nerve: the nerve then bifurcates in the orbit; the outer branch supplies the _Rectus externus_, the inner inosculates with small twigs of the ophthalmic division of the trigeminal, and then gives off a number of small ciliary nerves, already described, and one special branch to the _Retractor bulbi_.

*7.* The *facial nerve* (_N. facialis_, Ecker and Hoffmann; _Facialis_ (_Portio dura_), Wyman; Facial or Jugular Nerve, Stannius), (Figs. 111, 113, and 114 _Ve_ _VII_) arises immediately behind the trigeminal, and runs forwards, first in company with the auditory nerve, then alone, to the Gasserian ganglion (_q.v._), with which it unites; it again appears at the posterior angle of the ganglion, and escapes from the cranium in company with, and immediately behind, the _Ramus mandibularis_ of the trigeminal, and divides at once into two branches.

*a.* The _Ramus palatinus_ at once unites with the palatine branch of the ophthalmic division of the trigeminal nerve. Wiedersheim doubts this arrangement, and holds that the two portions arise in common from the Gasserian ganglion; neither has Wyman (_Rana pipiens_) described this branch of the facial. [De Watteville states that the _Ramus palatinus_ and the palatine branch of the ophthalmic nerve are separated by the carotid artery. In urodeles the two nerves are distinct.]

*b.* The _Ramus hyomandibularis_ (_Ramus jugularis_, Hoffmann; Facial Nerve, Wyman), (Figs. 113 and 114, _Ve_ _VII_). The nerve is directed outwards and backwards so as to pass around the bony wall of the auditory capsule, it then crosses over the inner end of the columella, with which it is in close contact, and is then joined, under cover of the _sterno-cleido-mastoideus_ (_sc_), by a branch (_Ramus communicans_) of the glossopharyngeal (see Fig. 111 *). The single trunk (_F_) so formed is directed downwards in the posterior wall of the Eustachian tube to just above the angle of the mandible, where it divides into three branches, or sometimes into two, in which case a third nerve is supplied by one of the others, usually by the _Ramus hyoideus_.

(1) The _Ramus mandibularis_ (_Ramus mentalis_, Hoffmann; Third Trunk of the Facial, Wyman), (Figs. 114 _mt_ and 115 _m_′) passes inside the angle of the jaw and courses forwards, between the skin and mylo-hyoid muscle, parallel to the _Ramus mandibularis_ of the trigeminal nerve, as far as the symphysis. It supplies small twigs to the neighbouring parts of the mucous membrane of the mouth.

(2) The _Ramus hyoideus_ (_Ramus jugularis_, Hoffmann; Second Trunk of the Facial, Wyman), (Figs. 114, 115 _h_) is the largest division of the facial; it courses forwards subcutaneously over the _deltoideus_ and the hinder fibres of the _mylo-hyoideus_ to the anterior cornu of the hyoid bone, supplying its muscles and the skin of the throat and sternal region.

(3) The _Ramus auricularis_ (First Trunk of the Facial, Wyman); is a small branch, directed outwards; it supplies a branch to the walls of the tympanic cavity, and is finally distributed in the skin under the tympanic membrane and behind the angle of the mouth.

*8.* The *auditory nerve* (_N. acusticus_, Ecker, Hoffmann; Auditory Nerve, Wyman). This nerve has a very short course in the cranium; it reaches the auditory organ by a foramen (Fig. 116 _VIII_) in the auditory capsule, and is then distributed in the ear, with which it will be described.

*9.* The *glossopharyngeal nerve* (_N. glossopharyngeus_, Ecker; _Ramus glosso-pharyngeus_, Hoffmann; Glossopharyngeal, Wyman; Glossopharyngeal of the Vagus, Müller), (Figs. 102 and 103 _X_, 111, 113, 114, and 115 _X_^2) arises in common with the pneumogastric nerve, and quits the skull with it through an opening (_Canalis nervi vagi_) immediately behind the auditory capsule, and at once divides into two branches; both of which lie under the _MM. intertransversarii capitis_ and are covered by the _Depressor mandibulae_.

*a.* The _Ramus anterior_ (Verbindungsast des Glossopharyngeus, Wiedersheim; _Ramus communicans_, Hoffmann; Uniting Branch, Wyman; Laryngeal Branch of the Vagus, Volkmann; _Communicans ad facialem_, de Watteville), (Figs. 111, 113, and 114 _X^1_). This nerve curves downwards and forwards around the auditory capsule and beneath the _Depressor mandibuli_ to join the facial nerve, as already described.

*b.* The _Ramus posterior_ (_N. glossopharyngeus_, Wiedersheim; _Ramus lingualis_, Hoffmann), (Figs. 111, 113, and 114 _X^2_). This nerve runs downwards and forwards to the ventral surface of the pharynx, dips underneath the _Petrohyoideus IV_, to appear again, after a short interval, by piercing the _Petrohyoideus II_ or _III_, courses parallel to and behind the anterior cornu of the hyoid bone, and thus reaches the floor of the mouth. The nerve then runs forwards in a very sinuous course, close to the median line, and between the _Geniohyoideus_ and _Hyoglossus_; in its course on the floor of the mouth it crosses the hypoglossal nerve. It supplies the petrohyoid muscles, and gives numerous small branches to the mucous membrane of the pharynx (_Ramus pharyngeus_, Hoffmann).

*10.* The *pneumogastric* or *vagus nerve* (_Vagus_; _Ramus intestinalis nervi vagi_, Fischer, Hoffmann; Vagal Trunk, Wyman; Vaso-sympathetic, Gaskell), (Figs. 111, 113, 114, and 115 _X_^3). This nerve arises in common with the glossopharyngeal; the two nerves leave the skull together by an opening in the exoccipital bone (_Canalis nervi vagi_); immediately outside the skull they acquire a ganglionic enlargement (_Ganglion condyloideum_, _Ganglion nervi vagi_), (Fig. 111 _XG_); in this course the glossopharyngeal lies in front of the pneumogastric, which it then leaves. The pneumogastric lies first upon the _M. levator anguli scapulae_, then running backwards and downwards along the hinder border of the _Petrohyoideus IV_ it comes to the side of the pharynx; it is covered by the trapezius, and passes between the hypoglossal nerve and the _Aorta ascendens_; arriving at the _Arteria pulmonalis_, it gives off its terminal branches. [Gaskell has proved that this nerve contains both sympathetic and vagal elements, and that it is therefore really a vago-sympathetic.]

Right half of skull of _Rana esculenta_, seen from within.

_II_ Optic nerve.
_III_ Motor oculi nerve.
_IV_ Trochlear nerve.
_VN_ Root of the trigeminal nerve.
_Vg_ Gasserian ganglion.
_VS_ Sympathetic nerve.
_VI_ Abducens nerve.
_VII^1_ Facial nerve.
_VIII_ Auditory nerve.
_X^{1–3}_ Pneumogastric and glossopharyngeal nerves.
]

The branches and communications of the pneumogastric nerve are:--

*a.* Communications between the ganglion of the pneumogastric and the sympathetic system (Fig. 111 _Sp_), which again connect it with the Gasserian ganglion of the trigeminal nerve (Fig. 111 _VS_, _Vg_); (see sympathetic system).

*b.* [The _Ramus cutaneus dorsalis_ (Fischer) is considered by Stannius and Fürbringer to be the homologue of the _Ramus auricularis_; it passes outwards between the _digastricus_ and _temporalis_ to be distributed in the skin of the suprascapular region. This nerve is the persistent portion of the _N. lateralis nervi vagi_ of the tadpole.]

*c.* During its course over the _petrohyoidei_ the vagus gives off a few twigs (Fig. 114 _pp_), which form a fine plexus to supply these muscles and the pharynx [also a twig to the trachea, according to Hoffmann].

*d.* The _Ramus accessorius_ (Fürbringer, Hoffmann, Ecker), (Fig. 114 _XI_) is usually a single small nerve which runs between the _intertransversarii_ muscles and the _trapezius_: it supplies the under surface of the latter muscle.

*e.* The _Ramus scapularis_ (Hoffmann) is a very slender nerve arising near the _Ramus accessorius_; it passes along the under surface of the _trapezius_ to the inner surface of the _interscapularis_, which it supplies.

*f.* The _Ramus laryngeus_ (_Recurrens vagi_), (Figs. 114 and 115 _X3l_) runs for some distance parallel to the pneumogastric, separated from it by the _petrohyoideus IV_; arriving at the hinder cornu of the hyoid bone the nerve loops round the _Arteria pulmonalis_, and divides into two branches for the supply of the larynx.

*g.* The _Rami gastrici_ (Fig. 114 _X3g_) are usually two in number: they pierce the partial diaphragm formed by the anterior fibres of the _M. obliquus internus_ and terminate in the walls of the stomach.

*h.* The _Rami pulmonales_ (Figs. 114 and 115 _X3p_) also perforate the partial diaphragm, and then course along the pulmonary arteries to the lungs.

*i.* The _Ramus cardiacus_ (Figs. 114 and 115 _X3c_). This nerve is usually smaller on the right side than on the left; it passes along the dorsal surface of the pulmonary artery and _Vena cava superior_ to the _Sinus venosus_; in this course it gives off two or three twigs to the roots of the lungs: the two nerves communicate just before reaching the heart and pass on to the auricular septum, whence they are distributed to the heart.

*j.* A slender *Laryngeal nerve* arises from the vagus alongside the _Ramus cardiacus_; it courses along the hinder end of the greater cornu of the hyoid to the outer side of the pharynx, which it pierces to pass to the larynx.

F. THE SPINAL NERVES.

I. *General description.* Ten pairs of nerves arise, as already described (p. 135), from the spinal cord; each nerve has two roots, a ventral or anterior, and a dorsal or posterior, which unite at their points of exit from the intervertebral foramen: just before, and for a short distance beyond this union, each dorsal root bears a ganglionic enlargement.

*a.* The length and direction of the various *Nerve-roots* vary greatly; the roots of the anterior spinal nerves run a very short course, almost transversely outwards, from their points of origin to the intervertebral foramina: the roots of the middle and posterior nerves, in consequence of the vertebral column being considerably longer than that part of the cord belonging to it, pass obliquely backwards to the foramina, the hinder nerves of the _Cauda equina_ running for a considerable distance in the vertebral canal (Fig. 116). The relations of these roots are as follows:--

(1) The roots of the first nerve (_N. hypoglossus_) arise from the spinal cord at a point between the first and second vertebrae; they run transversely outwards to escape between the first and second vertebrae. The dorsal root is extremely slender.

The nervous system of _Rana esculenta_, from the ventral surface. (From Icones physiologicae by A. Ecker. Pl. XXIV, Fig. 1.)

_F_ Facial nerve.
_G_ Ganglion of pneumogastric nerve.
_He_ Cerebral hemisphere.
_Lc_ Optic tract.
_Lop_ Optic lobe.
_M_ Boundary between medulla oblongata and spinal cord.
_M1–10_ Spinal nerves.
_MS_ Connection between fourth spinal nerve and
sympathetic chain.
_N_ Nasal sac.
_Ni_ Sciatic nerve.
_No_ Crural nerve.
_o_ Eyeball.
_S_ Trunk of sympathetic.
_S1–10_ Sympathetic ganglia.
_SM_ Rami communicantes of the same.
_Sp_ continuation of sympathetic into head.
_I_ Olfactory nerve.
_II_ Optic nerve.
_III_ Motor oculi nerve.
_IV_ Trochlear nerve.
_V_ Trigeminal and facial nerves.
_Va_ Ramus ophthalmicus.
_Vc_ Ramus maxillaris.
_Vd_ Mandibular branch of trigeminal.
_Ve_ Hyomandibular branch of facial.
_Vg_ Gasserian ganglion.
_Vs_ Upper end of sympathetic trunk in connection with
Gasserian ganglion.
_VI_ Abducens nerve.
_VII_ Facial nerve.
_VIII_ Auditory nerve.
_X_ Glossopharyngeal and pneumogastric nerves.
_X1_ Ramus anterior of glossopharyngeal.
_X2_ Ramus posterior of glossopharyngeal.
_X3, 4_ Branches of pneumogastric.
]

(2) The roots of the second nerve (_N. brachialis_) arise at the level of the second vertebra and leave the vertebral canal between the second and third vertebrae.

(3) The roots of the third nerve arise from the cord between the second and third vertebrae, and pass out between the third and fourth vertebrae.

(4) The fourth nerve arises by its two roots at the level of the fourth vertebra, and quits the vertebral canal between the fourth and fifth vertebrae.

(5) The fifth nerve arises opposite the fourth vertebra, and passes outwards and slightly backwards to the foramen between the fifth and sixth vertebrae.

(6) The roots of the sixth nerve are attached to the cord opposite the fourth vertebra, and leave the vertebral canal between the sixth and seventh vertebrae.

(7) The seventh nerve arises from the cord at a point between the fourth and fifth vertebrae, and leaves the canal between the seventh and eighth vertebrae.

(8) The eighth nerve quits the cord at a point opposite the articulation between the fifth and sixth vertebrae, then runs backwards to the eighth vertebra, and escapes between this and the ninth vertebra.

(9) The ninth nerve arises at the level of the sixth vertebra, and passes out between the sacrum (ninth vertebra) and the urostyle.

(10) The tenth nerve (_N. coccygeus_) arises from the cord immediately behind the ninth nerve and opposite the sixth vertebra, and courses alongside the terminal filament to pass out through a foramen in the urostyle (_canalis coccygeus_).

*b.* [The *Spinal ganglia* (_Ganglia intervertebralia_).

At their exit from the intervertebral foramen the two roots of each nerve unite and bear a ganglionic enlargement, which is in intimate connection with the dorsal root, but is merely in contact with the ventral root. The ganglia lie[58] in the large intervertebral depressions, upon the under surface of the _Proc. obliqui_, above and behind, being in relation with the bodies of the vertebrae internally, and the _Musculi intertransversarii_ externally (Fig. 119); ventrally the ganglia are, more or less, covered by the ‘periganglionic glands.’ The ganglia vary much in size; that of the first spinal nerve is the smallest. The ganglion is not attached to the dorsal root alone but is prolonged a short distance beyond the point of union of the two roots; this is best seen in the nerves of the lumbar plexus. Each ganglion is a yellowish-white rounded or oval body, with an outer thick (sometimes 0.15 mm. thick) sheath of connective-tissue, connected by the epineurium of the nerve roots with the _Dura mater_. This sheath is composed of bundles of parallel, wavy, white, connective-tissue fibres, which enclose a large number of connective-tissue corpuscles, and here and there contains brown pigment.

[Footnote 58: The _Gangl. coccygeum_ is in the urostyle.]

Ventral view of the brain and spinal cord, to show the points of exit of the spinal nerves.

_M1–10_ Spinal nerves.
_VG_ Trigeminal ganglion.
_W1–10_ Vertebrae.
_XG_ Ganglion of vagus.
]

By means of longitudinal and transverse sections it may be seen that the ganglion surrounds the ventral root, but that it forms a thinner layer on that side, where the dorsal root joins the ventral; although at this point the two roots and the ganglion are firmly united together, the thick fibrous sheath of the ganglion prevents any communication between the nervous elements of the ventral root and those of the ganglion.

The sheath contains nerve-cells and fibres, and sends in a few fine processes to support these structures. The fibres of the posterior root enter and pass out of the ganglion without suffering any changes, except that the nerve fibres are slightly separated by the presence of a few nerve-cells, and that a distinct increase in the number of nerve fibres takes place. The nerve-cells, of which the ganglion is chiefly composed, and upon the number of which its size directly depends, are arranged chiefly around the fibres of the dorsal root; such few as lie between the fibres are smaller than the rest and vary more in their relative numbers; at times one or two isolated cells are found in the capsule or even in the adjoining ‘periganglionic gland.’ The cells near the capsule are somewhat smaller than the deeper cells.[59] Each of these cells, which are usually pear-shaped, possesses only one process; the cell membrane is thick, resistant, and possessed of an external nucleated, endothelial covering, the space around being probably a pericellular lymph-space; it often contains one to three small fat-globules; the protoplasm of these has, according to v. Lenhossék, a concentric fibrillation; the nucleus is round, clear, and distinct, and relatively larger the smaller the nerve-cell; it is usually placed in the centre of the cell.

[Footnote 59: Larger cells of deeper layer 80 µ to 90 µ in diameter, sometimes one or two as large as 0·1 mm. v. Lenhossék.]

Ventral view of the spinal ganglia; on the right side they are still hidden by the ‘periganglionic glands.’ ]

Schema of spinal ganglion.

_A_ Ventral root.
_C_ Ramus communicans.
_G_ Ganglion
_O_ Dorsal division.
_P_ Dorsal root.
_R_ Ventral division.
]

In that portion of the cell towards the process is a portion brighter and less easily stained than the rest of the cell (Polarkernen, Courvoisier); this apparently possesses one or two nuclei, and is regarded as a cell by Lenhossék (Polarzellen); the process of each cell soon acquires a medullated sheath, and after a short course, 0.09 mm. to 0.35 mm., divides usually about the third constriction (Lenhossék). These processes all pass peripherally, even those which appear at first to pass in the opposite direction curve round again; it is chiefly due to these fibres that the spinal nerve is larger than its two roots.

In addition to the above, smaller, irregular cells from 5 µ to 7 µ in length are found; Rawitz regards them as young cells, Lenhossék as cells capable of (entwicklungsfähige) developing into nerve-cells. The ganglia contain only few blood-vessels.]

The above description is founded upon that of Lenhossék, and has been confirmed by the translator in every particular, except that only very indistinct and unsatisfactory indications of the concentric fibrillation so clearly delineated by Lenhossék were obtained; the structure of the Gasserian ganglion was found to correspond exactly with the description here given of the structure of the spinal ganglia. The nucleus was oftener at the side than at the centre of the cell.

*c.* The ‘*Periganglionic Glands*’ (Periganglionäre Kalkdrüsen, v. Lenhossék; Kalksäckchen, Ecker, and others; Crystal capsules, Wyman; Calcareous Sacs or Masses, of other writers), (Fig. 119). These bodies are found on the ganglia of all the spinal nerves and on the Gasserian ganglion of the trigeminal nerve; they have recently been carefully investigated by v. Lenhossék. According to his description there are usually two to each ganglion, lying on its sides and ventral surface; each consists of a connective-tissue capsule which sends in a few fine trabeculae to support a system of glandular tubes: these usually run parallel to the long axis of the gland, and are about 14 µ to 15 µ in diameter; each tube is lined with a single layer of somewhat columnar epithelium, the cells of which have sharp, distinct outlines and oval nuclei.

A _membrana propria_ was not made out with certainty; the lumen of the tubes is wide and contains a milky fluid, which gives the whole structure its characteristic appearance; it has long been known (Blasius, 1681, mentions the fact, _l.c._, p. 291) that this fluid contains calcareous matter, which effervesces and dissolves in the presence of hydrochloric acid; the crystals vary in form but are chiefly oval (Wyman).

These glands bear no definite proportion to the size of the nerves to which they are attached (Fig. 118); they already contain calcareous matter during the tadpole stage before the limbs are developed (Stannius, Wyman). No ducts have been traced to these glands. This description, founded upon that of Lenhossék, can be confirmed by the translator in every particular; his sections, however, would lead him to believe that the glands are far more vascular than the description and the drawings of Lenhossék imply.]

*d.* The *Branches* of the complete spinal nerves formed by the union of the dorsal and ventral roots with the ganglion (Fig. 120) are usually described as two, a dorsal and a ventral branch, which latter at once gives off a _Ramus communicans_ to the sympathetic system; these two branches are given off almost immediately beyond the ganglion.

II. The Individual Nerves.

A. The Dorsal Branches (Fig. 121).

Dorsal branches of the spinal nerves. The _Fascia dorsalis_ is dissected, removed from the left side.

_rc_ Rami cutanei.
_rm_ Rami musculares.
]

The dorsal branches are smaller than the ventral; they pass upwards between the inner borders of the _Musc. intertransversarii_ and the articular processes of the vertebrae to reach the under surface of the _M. longissimus dorsi_; the general course of these nerves is very similar for all; each nerve gives off--

(_a_) A twig to the _Musc. intertransversarius_ and then divides into two branches.

(_b_) A _Ramus muscularis_ (Fig. 121 _rm_), which passes outwards to supply the _M. longissimus dorsi_.

(_c_) A _Ramus cutaneus_ (Fig. 121 _rc_); this nerve continues under the extensor muscle of the back, running on the arch of the vertebra towards the middle line; it then pierces the muscle and Fascia dorsalis to reach the large dorsal lymph-sac (_Sacc. cranio-dorsalis_), through which it courses to end in the skin. In its course through the sac the nerve is connected with a small artery and vein by a small amount of connective-tissue, the whole being enclosed in a sheath of endothelium continuous with that of the general cavity.

The three anterior cutaneous nerves pierce the _Fascia dorsalis_ near the spinous processes, the fourth a little to the side, the fifth perforates the _M. coccygeo-iliacus_ at about the junction of its anterior and middle thirds; the sixth and seventh perforate the same muscle, but more posteriorly. There are seven of these _Rami cutanei_, the first coming from the second spinal nerve, the rest from the succeeding six spinal nerves.

B. The Ventral Branches.

The ventral branches of the spinal nerves are larger than their corresponding dorsal branches: each nerve gives off immediately a _Ramus communicans_ to the sympathetic system (Figs. 117 _SM_, 120 _C_); these _Rami communicantes_ will be described with the sympathetic system; the remaining portions of the ventral divisions of the original spinal nerves are usually known as the spinal nerves; they will be described as such in detail.

*1.* The *hypoglossal nerve*[60] (_N. hypoglossus_, _N. spinalis I_), (Figs. 114 115, and 117 _M1_), or first spinal nerve, is of small size; it leaves the vertebral canal between the first and second vertebrae to lie between the _Musculi intertransversarii_ and the pharynx, where it has in front the _Arteria vertebralis_, behind the _Arteria_ and _Nervus brachialis_; passing under the _Levator anguli scapulae_, it touches the _Aorta ascendens_ and crosses the _Vagus_ and the Carotid-gland, and thus reaches the space between the _Musculi sterno-hyoidei_ and the _Petro-hyoideus_; then curving forwards sharply it turns under the _Mylo-hyoideus_ to course between the fibres of the _Genio-hyoideus_ to the root of the tongue, where it ends. It first lies to the inner and then to the outer side of the glossopharyngeal nerve. In its course the nerve gives off the following branches:--

[Footnote 60: [Observers have differed considerably as to the relations of this nerve: Volkmann describes it as the first nerve of the neck, and describes a ganglion on its dorsal root; Vogt denies the presence of this ganglion, and regards it as a true cranial nerve; Stannius asserts that no ganglion exists, and holds the nerve to represent the first two spinal nerves; Wyman describes the two roots and regards it as a spinal nerve; Hoffmann regards this nerve as the second spinal (N. spinalis II); Fürbringer as the representative of two spinal nerves; de Watteville describes two roots and names it first spinal nerve.--TRANS.]]

(_a_) One or two communicating twigs (Figs. 114 and 122) to the second spinal nerve.

(_b_) Muscular twigs to the _Longissimus dorsi_, _Intertransversarii capitis_, _Levator anguli scapulae_, and the _Retrahens scapulae_.

(_c_) When near the glossopharyngeal it supplies twigs to the _Geniohyoideus_, _Sternohyoideus_, and _Omohyoideus_, where it bifurcates.

(_d_) One of the branches passes inwards to supply the _Hyoglossus_.

(_e_) The other terminal branch passes forwards with the _Ramus lingualis_ and the glossopharyngeal, between the fibres of the _Geniohyoideus_, which it partially supplies, and sends twigs to the neighbouring parts.

(_f_) [Hoffmann describes a communicating branch to the pneumogastric nerve, which the translator has not been able to discover, and which no other observer has mentioned.]

*2.* The *second spinal* or *brachial nerve* (_N. brachialis_, _N. spinalis II_), (Fig. 122) is a large nerve, leaving the vertebral canal between the second and third vertebrae; it then accompanies the _Art. axillaris_ along the anterior border of the transverse process of the third vertebra, over the hinder end of the _Musc. levator scapulae_ and the anterior end of the _m. transverso-scapularis major_, to the inner border of the _Musc. subscapularis_, under which it passes into the arm. In this course it gives off or receives the following branches:--

(_a_) It first receives a branch from the second spinal nerve.

(_b_) It gives off a large branch, the _N. coraco-clavicularis_ (Fig. 122 _Cc_), which first accompanies a branch of the _Art. axillaris_ on the _Musc. levat. scapulae_, then courses forwards and outwards over the _Musc. subscapularis_ to pass from above into the _Foramen ovale_ between the clavicle and coracoid bones. It gives off two branches:

(1) One immediately beyond its origin, which runs backwards over the _Musc. subscapularis_ to supply the _Musc. obliq. abdominis internus_ (Fig. 122 _Cc_′).

(2) While in the _Foramen ovale_, the _N. coraco-clavicularis_ bifurcates; the anterior twig runs forwards and outwards to the _deltoideus_ and also supplies a recurrent filament to the _Musc. sterno-radialis_; the posterior twig enters the upper surface of the _Musc. stero-radialis_.

(_c_) At the outer extremity of the _Musc. transverso-scapularis_ the brachial nerve gives off a posterior branch, which at once bifurcates:

(1) The first branch passes into the under surface of the _Musc. latissimus dorsi_ (_ld_), giving a twig to the _Infraspinatus_.

(2) The second is the _Ramus cutaneus axillaris_ (_IIc_); it passes on the under surface and posterior border of the _Musc. latissimus dorsi_ to the skin of axilla and dorsal surface of the upper arm.

(_d_) Several small branches are given off from the upper and anterior surfaces of the brachial nerve near the _Foramen ovale_; they accompany an artery to pass forwards and upwards between the _Musc. subscapularis_ and the _M. transverso-scapularis_ on the one side, and the long head of the _M. triceps brachialis_ on the other, and are lost in the _M. deltoideus_ and the _M. infraspinatus_.

The brachial plexus.

_Cc_ N. coraco-clavicularis.
_Cc_′ Nerve to musc. obliq. abdom internus.
_d_ Deltoid muscle.
_la_ Musc. lev. ang. scap.
_ld_ Musc. lat. dorsi.
_oi_ Musc. obliq. abdom. intern.
_sc_ Musc. sterno-mastoid.
_ss_ Musc. subscapularis.
_SI_ First spinal nerve.
_SII_ Second spinal nerve.
_SIII_ Third spinal nerve.
_IIc_ Ramus cutaneus axillaris.
_IIIc_ Ramus cutaneus abdominalis.
]

The _N. brachialis_ continues its course by passing between the long and inner heads of the _M. triceps_, reaches the arm and splits into two nerves, the _N. ulnaris_ and the _N. radialis_.

[*I.*] The _N. ulnaris_ (Fig. 123 _U_) escapes from cover of the long head of the _M. triceps_ and runs obliquely over its inner head towards the outer side of the _Plica cubiti_; in this course it supplies four branches:

α. The _Ramus subscapularis_ passes dorsally to the _M. subscapularis_.

β. A _Ramus pectoralis_ (Fig. 123 _up_), passes forwards to the _M. abdomino-pectoralis_ and to the skin of the pectoral region (_R. cutaneus pectoralis_).

γ. The _N. cutaneus antibrachii superior s. medialis_ (_uc_) passes to the skin of the inner side of the forearm; this and the two following nerves are given off near the _Plica cubiti_.

Nerves of the central surface of the arm. The hand pronated.

_rc_ Upper cutaneous branch of the N. radialis.
_rc_′ Lower cutaneous branch 4-,f the N radialis
_U_ N. ulnaris.
_uc_ R. cutan. sup. of the N. ulnaris.
_uc_′ R. cutan. inf. of the N ulnaris.
_up_ R. pectoralis of the N. ulnaris.
]

δ. The _N. cutaneus antibrachii inferior s. dorsalis s. musculo-cutaneus_ (_uc_′) at once supplies branches to the _M. flexor carpi radialis_[61], and then runs downwards upon the muscle and bifurcates:--

[Footnote 61: As this muscle increases in size during the breeding season. it would be interesting to know whether a corresponding change takes place in the nerve.]

[1] The _R. lateralis_ is distributed, by two twigs, in the _M. flexor carpi ulnaris_ and in the skin of the second finger, to which it supplies the _Rami digitales volaris_ and _dorsalis_.

[2] The _R. dorsalis_, after giving a cutaneous twig to the second finger, is distributed by numerous twigs to the skin of the dorsal surface of the hand.

The _N. ulnaris_ then passes between the _M. flexor carpi radialis_ and the tendon of the _M. sterno-radialis_, sinks deeply into the _Plica cubiti_, where it lies between the _M. flexor carpi radialis_ and the _Flexor carpi ulnaris_, then between the latter and the _Flexor anti-brachii medialis_ on the one side and the _Flexor digitorum communis_ on the other: it supplies branches to all these muscles, and while still in the forearm divides into two terminal branches:--

α. The _R. ulnaris medialis_ is the smaller; it runs inwards, passes under the tendon of the _M. extensor carpi ulnaris_ into the palm of the hand and ends as the _N. volaris digiti V medialis_.

β. The _R. ulnaris lateralis_ passes near the thumb-rudiment, covered by the _M. abductor pollicis_, deeply into the palm of the hand, and supplies by bifurcating branches the adjacent sides of the second, third, fourth, and fifth fingers (_R. volares_); it also gives twigs to the muscles of the palm.

[II.] The _N. radialis_ (Figs. 123, 124, and 125 _R_) passes immediately beyond its origin from the _N. brachialis_, between the humerus and the _M. triceps_, runs along the bone to its outer side; in this course it gives off:--

Nerves of the ventral surface of the arm. The hand supinated, and the superficial layer (Flex. carpi rad. and uln., Flex. dig. commun.) has been removed.

_R_ N. radialis.
_u_ N. ulnaris.
_uc_ R. cutan. inf. of the N. ulnaris.
_um_ Bifurcation of the N. ulnaris.
]

α. _Rami musculares_ to the separate parts of the _M. triceps_.

β. The _Ramus cutaneus superior_ (Fig. 125 _rc_) to the skin of the outer surface of the upper arm and forearm.

The nerve then sinks deeply under the origins of the _M. extensor carpi ulnaris_ and the _M. extensor digitorum communis_, where it bifurcates after giving off some small twigs:--

γ. Small muscular twigs to the extensor muscles of the hand and fingers.

The N. radialis.

_ec_ M. extensor carpi ulnaris.
_ed_ M. extensor digit. comm.
_R_ N. radialis.
_R1_ R. lateralis.
_R2_ R. medialis.
_rc_ R. cutaneus superior.
_rc_′ R. cutaneus inferior.
]

δ. The _R. radialis medialis_ (_R2_), the smaller terminal branch, supplies a branch to the _M. extensor digit. comm. longus_ and the skin over the carpus (_Ramus cutaneus inferior_, _rc_′), and ends on the muscles of the little finger as the _R. dorsalis digiti V medialis_.

ε. The _R. radialis lateralis_ (_R_), the larger of the two branches, supplies the extensor muscles of the fingers and gives bifurcating branches, _Rami digitales dorsales_, to the adjacent sides of the second, third, fourth, and fifth fingers (the _Ramus dorsalis lateralis_ of the second finger is supplied by the _N. ulnaris_).

*3.* The *third spinal nerve* (_N. spinalis III_) (Fig. 122 _SIII_) leaves the vertebral canal by the foramen between the third and fourth vertebrae, and runs outwards and forwards upon the large transverse process of the third vertebra to the brachial nerve (_N. spinalis II_). Its behaviour at this point is subject to considerable variation; at times it joins the brachial nerve completely, at other times the two nerves are simply in contact; or lastly, it may send a small twig to the brachial nerve. Externally to this point the two nerves supply a number of small branches, which may belong almost entirely to either one nerve or the other; these nerves again inosculate in a variable manner, to form an axillary plexus. The more constant branches are:--

(1) One or more twigs to the _Musc. transverso-scapularis major_.

(2) Several branches, usually two larger and one smaller; they course in the _M. obliq. abdom. internus_ (Fig. 122 _oi_) to the _M. rectus_, where they give off branches, then pierce the muscle to reach the skin (_R. cutaneus abdominalis_), (Fig. 122 _IIIc_).

*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.

Ventral view of the brain and spinal cord, to show the points of exit of the spinal nerves.

_M1–10_ Spinal nerves.
_VG_ Trigeminal ganglion.
_W1–10_ Vertebrae.
_XG_ Ganglion of the vagus.
]

*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.

The sciatic plexus, from a drawing by Wiedersheim.

_a_ Communicating nerve from the ninth spinal nerve.
_a_′ Nerve to musc. lev. ani.
_b_ Nerve to oviduct.
_c_ Urostyle.
_cc_ Communicating twig from the ninth spinal nerve.
_ci_ M. coccygeoiliacus.
_c.I_ M. coccygeosacralis.
_d_ Nerve to bladder.
_d_′ Twig to the m. iliacus.
_e_ Twig from sciatic plexus to coccygeal plexus.
_i_ M. intertransversarius.
_il_ M. ilio-coccygeus.
_ip_ M. ilio-psoas.
_lev_ M. lev. ani.
_oi_ M. obliq. abdom. internus.
_t_′ Transverse process of sacrum.
_Ao_ Aorta.
_Com_ Point of union of nerves _a_ and _cc_.
_Is_ Ischium.
_M5_ Fifth spinal nerve.
_M6_ Sixth spinal nerve.
_M7_ Seventh spinal nerve.
_M8_ Eighth spinal nerve.
_M9_ Ninth spinal nerve.
_M10_ Tenth spinal nerve.
_M7a_ M. iliohypogastricus.
_M7b_ M. cruralis.
_Ni_ Sciatic nerve.
_Ovd_ Oviduct.
_Rect_′ Rectum.
_S7_, _S8_, _S9_, _S10_ Rami communicantes of seventh,
eighth, ninth, and tenth spinal nerves.
_Ves_ Bladder.
8, 9 Eighth and ninth vertebrae.
]

(_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_.