This fact probably explains how the existence of "external ophthalmoplegia" and other distinct forms of orbital

paralysis may occur from organic lesions in the region of the

tegmentum.

The

TRIGEMINAL

NERVE resembles the spinal nerves in possessing both a motor and sensory root. The analogy is heightened, moreover, by the development of the ganglion of Gasser upon the sensory root. The deep fibers of this nerve demand detailed description, because they

a

take their origin from several nuclei (shown in Fig. 55).

1. Some fibers join the trigeminal nuclei of the medulla. The sensory nucleus is situated as high as the level of the point of escape of the nerve from the pons Varolii and is analogous, in the opinion of some authors of note, to the posterior horn of the gray matter of the spinal cord. It extends along the outer part of the floor of the fourth ventricle.

2. The so-called "ascending root of the nerve" is associated with the posterior columns of the spinal cord as low as the middle of the cervical region. Its fibers are found to pass through these columns from their origin in the posterior horns to reach the tubercle of Rolando in the medulla. The fibers of the ascending root of the trigeminus are associated with the sensory portion of that nerve.

3. The fibers of the so-called "descending root of the nerve" apparently arise (1) from the cerebellum; (2) from the locus cœruleus; (3) from a collection of cells in the lateral

DEEP ORIGIN OF FACIAL FIBERS.

339

wall of the aqueduct of Sylvius; and (4) from a large-celled motor root, in the region of the corpora quadrigemina. The descending root of the trigeminus is associated chiefly, if not exclusively, with the motor fibers of the nerve.

The ABDUCENS NERVE arises from a nucleus within the medulla composed of large cells and situated near to the junction of the medulla and pons at the bottom of a groove in the floor of the fourth ventricle. A communication probably exists between this nucleus and that of the motor-oculi nerve of the opposite side.

pmf n.ft

XII N.

fr

VIII p.n

r.b

s.g
Va

b..

a.f

8TH N.

P

VIII a.n

The FACIAL NERVE arises from a nucleus which appears to be a continuation of that of the abducens, but lying deeper in the substance of the medulla, and also from a nucleus (inferior facial nucleus) in the substance of the pons. Some fibers of the nerve descend in the crus cerebri; probably from the lenticular nucleus of the corpus striatum of the opposite cerebral hemisphere. The peculiar course of the facial fibers in the medulla is shown in Fig. 55. Gowers denies the statement, which was once accepted, that the facial nerve arises in part from the nucleus of the abducens nerve. The fibers of the facial nerve form a distinct loop, by doubling upon themselves, before escaping from the me

a.m.f

FIG. 75.-Section of the medulla at its upper part. a. m. f., anterior median fissure; p. m. f., posterior median fissure; 0, olivary body; s. g., substantia gelatinosa; f. r., reticular formation; P, anterior pyramid; Va, ascending root of fifth nerve; b and a, two bundles of auditory nerve inclosing r. b., the restiform body; VIII a. n., VIII p. n., anterior and posterior nuclei of the auditory nerve; n. f. t., nucleus of the funiculus tereles; XII N., nucleus of the hypoglossal nerve.

dulla. The relation of facial paralysis to lesions of the pons

has been discussed on page 291.

The AUDITORY NERVE arises from four nuclei within the substance of the medulla.

A small fasciculus escapes from the medulla between the facial and auditory nerves, called the "portio intermedia of Wrisberg." This is now considered as in no way related to the sense of hearing, but rather to that of taste. Although it passes, along with the auditory nerve, into the internal auditory canal, it subsequently joins the facial nerve and probably helps to form the chorda-tympani branch of that nerve.

The nuclei of the auditory nerve lie on a level with the broadest part of the fourth ventricle. They are termed the "anterior median," the "posterior median," the "anterior lateral," and the "posterior lateral." They have been already described in connection with the medulla. Some fibers of the auditory nerve have been traced to the superior vermiform process of the cerebellum (page 232).

Figs. 55 and 75 will show the relative situation of the four acoustic nuclei within the medulla. Some of the fibers of the auditory nerve decussate.

The NINTH or GLOSSO-PHARYNGEAL NERVE arises from a nucleus within the substance of the medulla that is not separated by a distinct boundary from that of the pneumogastric. Its situation and extent are shown in the drawing of Erb (Fig. 55).

The TENTH or PNEUMOGASTRIC NERVE arises from a nucleus within the medulla situated in its lower half in the floor of the fourth ventricle, and also from a nucleus in the vicinity of the olivary body. The latter is not shown in the diagrammatic drawings of Erb. The nerve also derives fibers of origin (see page 213) from the trineural bundle (Spitzka). As the nerve emerges from between the lateral column and the restiform body of the medulla, its roots, between twelve and fifteen in number, lie beneath those of the glosso-pharyngeal nerve. These roots join to form a flattened fasciculus that crosses the flocculus, in company with the glosso-pharyngeal nerve to reach the jugular foramen.

The ELEVENTH or SPINAL ACCESSORY NERVE is formed by

ORIGIN OF ELEVENTH AND TWELFTH NERVES. 341

two divisions, the medullary and the spinal. The former arises from a nucleus that lies within the medulla close to the central canal of the cord and is a continuation downward of the nucleus of the vagus. The spinal filaments spring from a continuation of that nucleus in the gray substance of the cord, as low down as the level of the escape of the sixth or seventh pair of cervical nerves. The spinal filaments escape from the lateral column of the cord, between the anterior and posterior nerve roots of the spinal nerves. The medullary portion of the nerve receives accessory fibers from the hypoglossal nucleus, the solitary bundle, and the raphe.

The TWELFTH or HYPOGLOSSAL NERVE arises from a nuclear column (about three quarters of an inch in length) that is composed of large branching multipolar nerve cells. Its lower end is situated in front of and close to the central canal, in the region of the level of decussation of the pyramidal tracts. In the fourth ventricle this nucleus forms a prominence near the median line, slightly above the calamus scriptorius. The filaments of the nerve pass through the inner side of the olivary body, and emerge in the furrow between the anterior pyramid and the olivary body. They are then collected into two bundles that join with each other to form the nerve trunk before they reach the anterior condyloid foramen.

We are now prepared to consider the separate cranial nerves and their branches of distribution.

THE OLFACTORY NERVE.

The first cranial nerve, or nerve of smell, consists (1) of three roots; (2) an olfactory process; (3) a bulb; and (4) terminal branches, which are distributed to the cavities of the

nose.

The three roots are called the external, middle, and internal.1

1 The deep origins of this nerve have been discussed in preceding pages, as well as the probable situation of the cortical centers of smell.

All three of the roots join to form a band, which is prismoidal in form (the olfactory process or tract), which passes forward

66

The numbers 1 to 15 refer to branches which will be described hereafter.

along the floor of the brain in a deep sulcus till it expands into the terminal enlargement, known as the "olfactory bulb," or ganglion." This terminal enlargement lies upon the upper surface of the cribriform plate of the ethmoid bone, through the numerous foramina of which its branches escape, as small, thread-like filaments; which subsequently form a plexus upon the surface of the Schneiderian, or pituitary, membrane of the nose.