2. The descending part of the medial longitudinal bundle is composed chiefly of uncrossed axones from the large cell-bodies in the nucleus tegmenti profundus of the reticular formation. This is the anterior reticulo-spinal fasciculus. Beginning at the nucleus tegmenti profundus, which is the nucleus lateralis superior of the reticular formation, it receives fibers from each reticular nucleus down to the nucleus lateralis inferior of the medulla. It receives the largest accession of fibers in the pons, where the nucleus lateralis medius and the three nuclei centrales are located. On this account James S. Collier suggests that it be called the medial ponto-spinal tract. It has been traced through the anterior fasciculus proprius to the lower part of the spinal cord. Its size is gradually reduced by the ending of a few fibers in the gray substance corresponding to each segment of the cord. It is associative in function.
Muskens and others describe a second descending strand in the medial longitudinal bundle. Muskens calls it the commis suro-medullary fasciculus. It rises from two nuclei: the nucleus of the posterior commissure (nucleus of Darkschewitsch) and the nucleus of the medial longitudinal bundle. The nucleus of the posterior commissure is in the prolongation of central gray substance, ventral and frontal to the posterior commissure; the nucleus of the medial longitudinal bundle lies still farther for ward in the lateral wall of the third ventricle, just caudal to the plane of the mammillary bodies. The fibers rising in these nuclei are small and descend, with few exceptions, no farther than the abducent nucleus; a very few have been found in the cervical cord.
The medial longitudinal bundle is an association tract of great importance in all vertebrates, especially connecting the ves tibular nuclei with the nuclei controlling ocular movements. It contains fibers from Deiter's and Bechterew's nuclei on the same side and from the chief nucleus of Schwalbe on the opposite side. The fibers divide T-like and the branches ascend and descend to reach the motor nuclei of the brain-stem. These fibers are medullated in the fourth and fifth months with the fasciculus proprius of the cord.
The lateral reticulo-spinal fasciculus begins to form in the lower part of the tegmentum. It is composed of crossed fibers from the nucleus tegmenti profundus in the mid-brain. Aug mented by axones from the other reticular nuclei in the pons and medulla, it continues downward through the lateral column of the spinal cord. Many axones of the reticular nuclei divide
into an ascending and a descending ramus; there are also ascending fibers in the tract whose origin is in the cord. The latter form a spino-reticular fasciculus.
In the mid-brain the medial longitudinal bundle also contains fibers derived—(r) from the oculomotor nucleus, which descend to the pons, and enter into facial nerve through which they supply the muscles of expression above the eye; and (2) from the nucleus of the abducent nerve. Running upward and decussating, the latter strand of fibers terminates in the opposite nucleus of the oculomotor nerve, and thus innervates the medial rectus of that eye. This strand accounts for the conjugate action of the two eyes in both health and disease.
Anterior Tecto-spinal Fasciculus (anterior or ventral longi tudinal bundle of Heald).—The anterior tecto-spinal bundle runs ventro-lateral to the medial longitudinal bundle and so close to it as to be considered a part of that bundle by many anatomists. It is not a separate and discrete bundle visible in the normal adult brain; it has been located by a study of mye linization and of degeneration following lesions of the tectum (quadrigeminal lamina) (see Figs. 6o and 64). A few of its fibers rise in the inferior colliculus but its chief origin is in the superior colliculus of the corpora quadrigemina, whence it decussates at once through the dorsal tegmental decussation (Meynerti) and descends through the reticular formation of the pons and medulla; and then through the fissural side of the anterior column of the spinal cord until it fades away in the lumbar region. Its fibers end in the gray matter of the cord on both sides and in the genetic nuclei of cerebral nerves; but chiefly in the nuclei of the oculomotor, trochlear and abducent nerves and in the cilio-spinal center of the spinal cord. In function the anterior tecto-spinal bundle is reflex; it is connected with all ocular reflexes which are excited by impulses from the retinm, such as accommodation for distance, pupillary contrac tion and dilatation.
The lateral tecto-spinal fasciculus rises in both quadrigeminal colliculi on the same side. It descends without crossing through the ventro-lateral part of the tegmentum, just dorsal to the medial fillet which is about to be described. In its course through the pons, medulla and cord it is associated with the rubro-spinal and thalamo-spinal tracts; like them it establishes connections with motor nuclei.