ventricle, this nucleus creates a prominence close to the median line of the ventricle, slightly above the point of the calamus scriptorius, or the lower angle of the ventricle.

FIG. 57.-A cross-section of the medulla oblongata on a level with the superficial origin of the auditory nerve. (Modified from Flechsig by Ross.)

ip and ep, the internal and external divisions of the inferior peduncle of the cerebellum; frs, reticular formation; a, arciform fibers; R. VIII, root of eighth nerve; VIII and VIII", nuclei of same; H, hypoglossal nucleus; P, pyramid; p, accessory portion of same; po, parolivary body; np, nucleus of the pyramid; at, ascending root of fifth nerve; de, direct cerebellar tract; L, posterior longitudinal bundle; 0, olivary body; sg, substantia gelatinosa.

That the fibers of the hypoglossal nerve are prolonged in some way upward to the cerebral cortex seems to be proven by pathological facts; although they have not yet been traced to such a termination.

The medullary portion of the spinal accessory nerve is destined to join the pneumogastric nerve after its exit from the skull, while the spinal portion pursues a separate course.

MOTOR NUCLEI OF MEDULLA.

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Its nucleus seems to be a prolongation downward of the nucleus of the vagus, as its course would naturally suggest.

In a general way, it may be stated that the nuclei of those cranial nerves within the medulla which are motor in function are characterized by large multipolar cells, while those which are connected with the sensory nerve-roots are composed of cells of smaller size, some of which are bipolar. In some respects they are analogous respectively to the cells of the motor or kinesodic, and the sensory or æsthesodic regions of the spinal gray matter; and also to the gray substance of those convolutions of the cerebrum which preside over similar functions.

Spitzka has called attention to the fact that the evolution of nervous force does not necessarily imply the existence of distinct fibers and of nerve cells. He says: "In the lower forms of animal life a uniform blastema, with or without nuclei, and entirely devoid of nerve cells, is frequently all that represents a ganglion; and the afferent and efferent strands connected with such a simple structure are composed of an infinite number of granules which show no fibrillary arrangement."

Again, the same author remarks: "The increased perfection of the nervous system is marked by a progressive tendency to isolation of the conducting strands."

THE UPWARD CONTINUATION OF THE ANTERIOR HORNS OF THE SPINAL GRAY SUBSTANCE.-We have already mentioned a change in the anterior horns' at the lower part of the medulla produced by the decussation of the motor fibers, which compose the so-called "crossed pyramidal tracts." The formation of two groups of cells, called the antero-lateral and the postero-lateral groups, here takes place. Portions of these groups are carried into the lateral column of the medulla, and form the so-called "anterior" and "posterior

1 The nucleus of the pyramid is to be regarded as “the amputated part of the anterior horn" of the corresponding side of the cord. Spitzka regards it as an accessory nucleus to the hypoglossal nerve, classing it in the same category with scattered gray masses from the lateral system higher in the medulla. He believes it to be the nucleus of origin of those fibers which supply the retractors of the hyoidean apparatus.

nuclei" of that column. The same arrangement appears to be carried out as we study ascending sections, so that the spinal accessory, the motor fibers of the pneumogastric and glosso-pharyngeal, the facial, and the motor fibers of the trigeminus seem to spring from a continuation upward of the anterior horns of the spinal gray substance in its lateral column.

The fibers of these nerves take, as a rule, an extremely circuitous course before escaping from the medulla. In the case of the facial nerve, a peculiar bend in its fibers of origin has been named the "knee" ("genu nervi facialis").

The anterior nucleus of the lateral column of the medulla appears to cease on a level with the origin of the facial nerve, although some authors believe that some fibers of the trigeminus pass from it in an upward and backward direction.

FIG 58.-A transverse section through the pons, on a level with the roots of the sixth and seventh cranial nerves from a nine months' embryo. (Modified from Erb and Ross.)

The right half represents a section made a little lower than the left. Tr, transverse fibers of the pons; P, pyramidal fibers; so, superior olivary body; L, posterior longitudinal fasciculus; t, fasciculus teretes (round bundle); RVI, root of abducens; RVII, root of facial; at, ascending root of trigeminus; R, round bundle; B, peduncle of cerebellum; ar, upward prolongation of anterior root zone of the spinal cord; alc, anterior nucleus of facial nerve; ple, posterior nucleus of same. This figure shows well the interlacing of the vertical pyramidal fibers with the horizontal (transverse) fibers of the pons.

THE HYPOGLOSSAL NUCLEUS.

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The posterior nucleus of the lateral column of the medulla passes behind and to the outside of the nucleus of the abducens, or sixth nerve, before joining with that of the facial.

The development of the olivary body within the substance of the medulla tends to displace the whole of the gray matter backward, until the posterior gray commissure (Fig. 52) disappears, and the central canal of the cord opens on the floor of the fourth ventricle.

The nucleus for the hypoglossal nerve roots bears a striking analogy, as regards the peculiar distribution of its cells, to the anterior horns of the spinal gray matter. The analogy is heightened furthermore by an identical method of development of these groups.'

At the upper level of the hypoglossal nucleus the transverse fibers of the pons tend to separate those masses of gray matter, which are probably an extension upward of the anterior horns of the spinal gray substance, or the gelatinous substance anterior to the central canal. Hence it becomes extremely difficult to trace their connections.

In the larger nucleus of the hypoglossal nerve, Spitzka describes a collection of cells whose axis-cylinder processes run away from the tract of the hypoglossal fibers, rather than toward them. These cells he believes to be associated with the eleventh nerve roots; and he thinks that an associating mechanism is thus established between the movements of the tongue and of the vocal cords, as is required in phonation and singing.

There are some grounds for the belief that the nucleus for the abducens, or sixth nerve, is a direct continuation of the postero-lateral group of cells; because the fibers of the facial nerve wind around it as those of the spinal accessory nerve do around the postero-lateral group in the lower part of the medulla.

1 Spitzka makes use of the following general deduction or axiom: "A ganglion (center) follows in development the development of the periphery, which is projected in that ganglion." He advances many interesting facts in support of this law, from a study of the comparative anatomy of animals. "Journal of Nervous and Mental Diseases," October, 1879, p. 616.

The nucleus for the motor-oculi, or third nerve, which has been partly described in the pages that treat of the crus cerebri, probably belongs to a group of cells which also constitutes the nucleus of the abducens nerve. In point of fact, it is reasonable to consider the nuclei of the third, fourth, and sixth cranial nerves as parts of one mass of

FIG. 59.-A transverse section of the medulla (partly schematic) made through the middle of the olivary body. (Modified from Spitzka.)

H, and h, nuclei of origin of the hypoglossal nerve (twelfth cranial); F. R., reticular formation, with its cell-masses; 0, olivary body; P, pyramid; a. m. f., antero-median fissure; G and Pn, masses of cells probably associated respectively with the glosso-pharyngeal and pneumogastric nerves; Va, ascending root of fifth cranial nerve; B, restiform column; a. f., arcuate fibers; F, fibers passing through the interolivary tract; e and d, bundles of fibers from the posterior spinal tracts, cut across on their way to the inferior cerebellar peduncle after decussation; T, the "trineural fasciculus" of Spitzka; "solitary" or "round" bundle of other authors. Note that the solid masses are composed of cells; the black areas are designed to represent conducting fibers running vertical to the plane of the section; the white lines represent fibers which run in the plane of the section.

cells, which have become separated by the interposition of some fibers destined to play an important but unknown part in the varied functions of the medulla. These nuclei all act upon the eye. They are connected, according to some authors, by distinct fibers, which run from the abducens nucleus to that of the third nerve of the opposite side.