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FIG. 662.-THE DISTRIBUTION OF THE PNEUMOGASTRIC NERVE.
Va, Right and left vagi; r, Ganglion jugulare and connexions with Sy, Sympathetic, superior cervical ganglion; G.Ph, Glossopharyngeal; Acc, Accessory nerve; F, Meningeal branch; Aur, Auricular branch; Va, Connexion with ganglion nodosum of vagus; Sy, Nerve to stylo-hyoid; Hy, Nerve to hyoglossus; C1, C2, Loop between the first two cervical nerves; Sy, Sympathetic, superior cervical ganglion; Acc, Accessory nerve; Ph. Pharyngeal branch; Ph.Pl, Pharyngeal plexus; S.L, Superior laryngeal nerve; I.L, Internal laryngeal branch; E. L, External laryngeal branch; I.C, Internal, and E.C, External carotid arteries; Cal, Superior cervical cardiac branch; Ca2, Inferior cervical cardise branch; R. L, Recurrent nerve; Ca3, Cardiac branches from recurrent nerves; Ca4, Thoracic cardiac brauch (right vagus); A.P.P, Anterior, and P.P.P, Posterior pulmonary plexuses; Oes. Pl, Esophageal plexus; Cort. Pl, Coeliac plexus.
the lateral aspect of the spinal medulla, between the anterior and posterior roots of the spinal nerves, its origin extending from the level of the accessory portion as low as the origin of the sixth cervical nerve (for the deep origin, see p. 596). Successively joining together, the fila radicularia (rootlets) form a trunk which ascends in the subdural space of the spinal medulla, posterior to the ligamentum denticulatum, to the foramen magnum. There the two portions unite into a single trunk, which leaves the cranial cavity through the jugular foramen in the same compartment of dura mater as the vagus nerve
(Fig. 647, p. 771).
Ramus Internus.-In the jugular foramen the accessory portion of the nerve or internal ramus (after furnishing a small branch to the jugular ganglion of the vagus) applies itself to the ganglion nodosum, and in part joins the ganglion, in part the trunk of the nerve beyond the ganglion. By means of these connexions the vagus receives viscero-motor and cardioinhibitory fibres.
Ramus Externus.-The spinal portion of the nerve, or external ramus, extends into the neck, where at first it lies along with other nerves, in the interval between the internal carotid artery and the internal jugular vein. Passing obliquely downwards and laterally over the vein, it descends close beneath the sterno-mastoid muscle, which it supplies as it pierces it on its deep surface. After crossing the posterior triangle the nerve ends by supplying the trapezius muscle on its deep surface. This portion of the nerve communicates in three situations with nerves from the cervical plexus-(1) in or beneath the sterno-mastoid, with the branch for the muscle
NEXIONS, AND DISTRIBUTION OF THE
derived from the second cervical nerve; (2) in FIG. 663.-SCHEME OF THE ORIGIN, CONthe posterior triangle, with branches from the third and fourth cervical nerves; (3) beneath the trapezius, with the branches for the muscle derived from the third and fourth cervical nerves.
Acc, Accessory nerve; C.1-4, First four cervical nerves (posterior roots); Va, Vagus nerve; R, Ganglion jugulare; T, Ganglion nodosum; G.Ph, Glossopharyngeal nerve; S.M, Nerves to sternocleido-mastoid; Tr, Nerves to trapezius; F.M, Foramen magnum; J. F, Jugular foramen.
The twelfth or hypoglossal nerve arises by numerous fila radicularia from the front of the medulla oblongata between the pyramid and the olive (Fig. 643, p. 768) (for the deep origin, see p. 594). The fila arrange themselves in two bundles which separately pierce the dura mater, and unite in the hypoglossal canal, or after emerging from the skull. In the neck the nerve arches downwards and forwards towards the hyoid bone, and then turns medially among the supra-hyoid muscles to the tongue. At first it is placed deeply, along with other cerebral nerves, on the lateral side of the internal carotid artery; it then curves forwards and downwards over the two carotid arteries, lying medial to the digastric and stylo-hyoid muscles. As it crosses the external carotid artery it hooks round the occipital artery. Above the greater cornu of the hyoid bone the nerve conceals the lingual artery; and it then disappears between the mylo-hyoid and hyoglossus muscles to reach the tongue, in the muscular substance of which it terminates.
Communications.—In its course the hypoglossal nerve has the following communications with other nerves: Near the base of the skull it is connected by small branches with (1) the superior cervical ganglion of the sympathetic; (2) the ganglion nodosum of the vagus; (3) by a larger branch, with the loop between the first two cervical nerves; (4) as it crosses the external carotid artery it receives a communication from the
pharyngeal plexus (lingual branch of the vagus); and (5) medial to the mylo-hyoid muscle, at the anterior border of the hyoglossus, it forms loops of communication with the lingual branch of the mandibular nerve.
The branches of the nerve are:-(1) Recurrent; (2) Descending; (3) Thyreohyoid; and (4) Lingual.
Ramus Recurrens.-The recurrent branch passes from the nerve near its origin to supply the dura mater of the posterior fossa of the base of the skull. It probably derives its fibres from the communication with the first and second cervical nerves.
FIG. 664.-THE MUSCLES OF THE HYOID BONE AND STYLOID PROCESS, AND THE
Ramus Descendens. The descending branch of the hypoglossal nerve is the chief branch given off in the neck. It arises from the hypoglossal nerve as it crosses the internal carotid artery, and descends in the anterior triangle superficial to the carotid sheath. It is joined about the middle of the neck by the descending cervical nerve (from the second and third cervical nerves). By their union the ansa hypoglossi (hypoglossal loop) is formed, from which branches are distributed to the majority of the infra-hyoid muscles-both bellies of the omo-hyoid, the sternohyoid, and the sterno-thyreoid. The descending branch of the hypoglossal nerve derives its fibres from the communication to the hypoglossal nerve from the loop between the first and second cervical nerves; so that the ansa hypoglossi is made of fibres of the first three cervical nerves.
Ramus Thyreohyoideus. The nerve to the thyreo-hyoid muscle is a small branch which arises from the hypoglossal nerve before it passes medial to the mylohyoid muscle. It descends behind the greater cornu of the hyoid bone to reach the muscle. When traced backwards this nerve is found associated with the loop between the first and second cervical nerves.
Rami Linguales.-The lingual branches of the hypoglossal nerve are distributed to the hyoglossus, genio-hyoid, and genioglossus, and to all the intrinsic muscles of the tongue. The nerve to the genio-hyoid is said to be derived from the loop between the first and second cervical nerves. It is not known if these two cervical nerves are implicated in the innervation of the proper muscles of the tongue, but it appears certain that the muscles named-the genio-hyoid, thyreo-hyoid, sternohyoid, omo-hyoid, and sterno-thyreoid-are not supplied by the hypoglossal, but only by cervical nerves, the genio-hyoid and thyreo-hyoid by the first two, the other muscles by the first three cervical nerves.
THE MORPHOLOGY OF THE CEREBRAL NERVES.
The head and face, possibly the oldest, and from every point of view the most fundamental and important portion of the body fabric, present in some respects a more conservative type of
FIG. 665.-COMPARISON OF ORIGINS OF NERVE ROOTS FROM SPINAL MEDULLA AND HIND-BRAIN (after His).
A. Spinal medulla; B. Hind-brain.
structure, and in other aspects have been subject to more profound alterations than other parts of the body. Segmentation is characteristic of the trunk, pervading bones, muscles, vessels, and nerves. An absence of true segmentation is characteristic of the head region-omitting for the moment the cerebral nerves. The head is characterised by the possession of an unsegmented tubular nervous system, enclosed in a bony capsule not obviously segmental, with which the capsules of the sense-organs become united. The pre-oral and post-oral visceral arches and clefts are not truly segmental like the costal arches of the trunk. The branchial clefts are said to be inter-segmental; and their muscles (associated with the myoblast surrounding the developing heart) are described as visceral, and not myotomic, so that the branchial vessels and nerves (similarly) are not to be regarded as comparable to the segmental vessels and nerves of the trunk. The truly segmental structures present are certain persistent myotomes or muscle plates, which give rise to muscles innervated by the third, fourth, sixth, and twelfth cerebral nerves.
Another difficulty in the morphology of the head arises from the absence of body cavity, and the consequent difficulty of differentiating the somatic and splanchnic mesoderm, and the somatic and splanchnic distribution of a given nerve.
Under these circumstances there is little help to be derived from head structures other than the nerves themselves in seeking a solution of the question of the morphological relations of the cerebral nerves. The spinal nerves are, generally speaking, all alike. The cerebral nerves, on the other hand, are all different. Scarcely any two nerves are alike; and no single cerebral nerve possesses in itself all the characteristic features of a spinal nerve. As seen in the account of its development (p. 504), the cranial nervous system possesses a series of dorsal ganglia, comparable in position and development to the spinal ganglia with which afferent nerves are associated; and the efferent roots are developed in the same way, and occupy somewhat the same position
as the anterior roots of the spinal nerves. But there is no single complete segmental nerve in the head. The very essence of the architecture of the head is a want of segmentation; and this character is shared by the cerebral nerves. In addition it must be borne in mind that, in relation to the mammalian head, there are organs which have no homologues in the trunk, and on whose existence the essential arrangement of the cerebral nerves depends-e.g. sense-organs and gill-arches.
Among the cerebral nerves there are several which possess a resemblance to one or other of the elements of a typical spinal nerve. In the neck the origin of the fibres of the accessory nerve is from the side of the spinal medulla, and it is in series with the motor roots of the vago. glossopharyngeal, facial, and fifth nerves. His (as shown in the account of the develop ment of the nerves) has described the neuroblastic origin of the motor roots of these nerves from the lateral part of the basal lamina of the primitive brain. They thus form a series apart-lateral motor roots-separable from the series of motor roots originating from the medial part of the basal lamina, comprising those of the third, fourth, sixth, and twelfth nerves; the latter nerve roots being comparable to and in series with the anterior roots of the spinal nerves. The lateral motor roots are not represented in the spinal series except in the neck. It is questionable if there is any fundamental distinction between the lateral and anterior motor roots of the cerebral nerves. The accessory fibres, for example, when traced into the spinal medulla, have an origin from the anterior column of the spinal medulla, and only differ from the motor or anterior root fibres of a spinal nerve in their different course to the surface. The ganglia in associa tion with the cerebral nerves are comparable to the spinal ganglia. The trigeminal nerve, with the semilunar ganglion, the ganglion of the facial, the ganglia of the acoustic, of the glossopharyngeal and the vagus, and the transitory (Froriep's) ganglion of the hypoglossal nerves, arise from the brain in a comparable position, and in the same way as the spinal ganglia. But another series of structures-the sense organs of the lateral line, and the so-called "epibranchial" organs which are highly developed in lower vertebrates (e.g. elasmobranchs), and which appear transitorily only, or are absent altogether in mammalian development, may possibly have a share in the formation of certain of these ganglia or parts of them (e.g. ciliary ganglion, genicular ganglion, ganglia of the acoustic nerve, petrous ganglion of the glossopharyngeal, and the ganglion nodosum of the vagus).
Certain of the cerebral nerves are apparently distinctly segmental, supplying muscles derived from the persisting myotomes of the head. The first three myotomes are said to give rise to the muscles of the eyeball. The first produces the superior rectus, inferior rectus, medial rectus, and inferior oblique muscles, and its segmental nerve is the oculo-motor. The second myotome is said to produce the superior oblique muscle, and its segmental nerve is the trochlear. The third myotome is said to produce the lateral rectus muscle, and its segmental nerve is the abducent. It has been asserted that the tongue muscles are derived from the last three or four cephalic and first cervical myotomes, and that the hypoglossal nerve is the segmental nerve for these myotomes, comprising the motor elements of several (four or five) segmental nerves. The intervening myotomes between the first three and this occipital series disappearing, the corre sponding elements of segmental nerves are supposed to be absent also (Fig. 666).
Certain of the cerebral nerves are essentially related to the structures derived from and asso ciated with the pre-oral and post-oral visceral clefts and arches (Fig. 667). The trigeminal nerve is essentially the nerve of the mandibular arch. By its efferent root it supplies the muscles of that arch. By its afferent root and branches it is related to (1) the fronto-nasal process (ophthalmic division and ciliary ganglion); (2) the maxillary arch (maxillary nerve); and (3) the mandibular arch (mandibular nerve). The mandibular is at first the main nerve; and the maxillary division is sometimes regarded as a subordinate branch (præ-branchial, præ-trematic for the supply of the anterior margin of the cleft (mouth), with which the nerve is in relation. The ophthalmic nerve is sometimes regarded as a morphologically separate nerve. The nerves to these arches have been compared to the anterior rami of spinal nerves, the branches which they supply to the forehead and temple (frontal, zygomatic, and auriculo-temporal) representing the posterior rami. The ganglia on each division of the nerve are formed as extensions from the semilunar ganglion.
The facial nerve is essentially the nerve of the second (hyoid) arch and the cleft in front of that arch (spiracular cleft, auditory tube). Its motor root supplies the muscles of that arch (stapedius, stylo-hyoid, and digastric), and the epicranial and facial muscles and platysma, which are developments from the hyoid arch (Rabl). The chorda tympani nerve is regarded as the subordinate (præ-branchial, præ-trematic) branch to supply the anterior margin of the first post-oral cleft. It is possible that the genicular ganglion, with the nervus intermedius and the chorda tympani, may, in part at least, represent the ganglionic and afferent element of the nerve. Or the genicular ganglion, and the nerves in relation to it, may be associated with an "epibranchial" sense-organ.
The acoustic nerve, on the other hand, may be either the sensory element of the branchial nerve, associated with the hyoid arch and first post-oral cleft, or it may represent the nerve or nerves belonging to ancestral sense-organs of the lateral line.
The glossopharyngeal is the branchial nerve of the third post-oral (thyreo-hyoid) arch and the cleft in front. Its efferent fibres supply the muscle of this arch, the stylopharyngeus The superior constrictor of the pharynx is also assigned to this arch; the middle and inferior muscles to the fourth (first branchial) arch. The afferent portion of the nerve is possibly composed of two separate parts; the petrous ganglion being associated with an epibranchial or lateral line sense-organ, and the rest of the nerve forming the afferent fibres for the gill-cleft