Cerebrum Surfaces

fibers, nucleus, longitudinal, bundle, dorsal, nuclei and medial

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The opposite pyramidal tracts and probably the three homo lateral, cerebro-pontal paths (fronto-pontal, temporo-pontal and intermediate paths) bring these nuclei into relation with the cerebral cortex; and the anterior and the medial longitudinal bundles establish their reflex relation.

Fasciculus Longitudinalis Dorsalis of Schutz (Bechterew's dorsal longitudinal bundle of the central gray substance, dorsal gray longitudinal bundle of KoHiker). The dorsal longitudinal bundle of Schutz rises in the central gray sub stance and its dorsal tegmental nucleus. In the form of a thin, broad tract it courses downward through the gray substance, near the ependyma, as far as the spinal cord; its fibers are believed to terminate in the cranial nerve and other nuclei. The tract probably belongs to a very primitive reflex mechanism (olfactory).

Formatio Reticularis (Fig. 6o).—Through the greater por tion of the tegmenta there are many oblique fibers interwoven with tracts of longitudinal fibers so as to produce a reticulum or net. Imbedded in the reticular formation, ventro-lateral to the oculomotor nucleus, lies its superior lateral nucleus, called the nucleus tegmenti profundus. This is the only nucleus in the reticular formation of the mid-brain. It is composed of a medial and a lateral part. In the stratum griseum centrale and dorsal to the trochlear nucleus, the nucleus tegmenti dorsalis is located. It extends a short distance into the pons. The reticular formation contains one large nucleus of pinkish color, the nucleus ruber, which is clearly visible near the median raphe and the substantia nigra; its outline is definite because it is surrounded by medullated fibers. Many of the oblique fibers of the formatio reticularis pass through the median raphe into the opposite tegmentum; they produce the teg mental decussations, which are situated at three successive levels, viz., the superior colliculus, the inferior colliculus and the isthmus rhombencephali.

The tegmental decussations at the level of the superior quad rigeminal colliculus (the fountain* decussations) are: (1) The dorsal tegmental decussation (Meynerti) through which the tecto-spinal or anterior longitudinal bundle crosses. It is situated between the red nuclei but dorsal to them. (2) The decussation of the fasciculus mammillo-tegmentalis.

(3) The ventral tegmental decussation (Foreli) in which the tract from the red nucleus, the rubro-spinal tract, crosses to the opposite side (Fig. 59).

At the level of the inferior colliculus (Fig. 6o ) is the decussa tion of the brachium conjunctivum (decussatio brachii conjunc tiva). It crosses with its mate to reach the opposite red nucleus.

At the level of the isthmus is located the vestibular corn missure composed of fibers which connect the vestibular nuclei of the auditory nerve. Intermingled with the above bundles of crossing fibers there are many other fibers of the formatio reticularis.

Tracts of Fibers in the Tegmentum (Figs. 58, 63 and 64).— In the reticulum of the tegmentum there are many bundles of longitudinal fibers, viz., the medial longitudinal, tecto-spinal (anterior and lateral), gustatory, rubro-spinal, thalamo spinal, thalamo-olivary, spino-thalamic, spino-t ectal, fillet, branchium conjunctivum, and mesencephalic root of the trigem inal nerve.

The medial (or posterior) longitudinal bundle (fasciculus longitudinalis medialis, Fig. 58) is a compact strand of fibers running along the median raphe just ventral to the central gray substance. In Weigert-Pal sections it shows clearly as a broad dark triangle 1-2 mm. thick next the raphe. The tract will be found in the same relative position in the pons and medulla oblongata. In addition to several very small strands of fibers which will be explained later, the medial longitudinal bundle is functionally composed of two tracts: (1) An afferent bundle; and (2) an efferent bundle.

1. The part of the medial longitudinal bundle is composed of T-branched fibers derived from the gray matter of the spinal cord and from the terminal nuclei of sensory cerebral nerves, especially from the vestibular nuclei of both sides. It is the continuation of the long ascending fibers of the anterior fasciculus propius. Possibly a small portion of the tract runs through the posterior commissure to the thalamus and is common sensory in function; but the major part of it decussates in several successive strands which end in the opposite motor nuclei of the cerebral nerves. The function of this latter part is reflex and coordinating.

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