The division of the pyramidal tract into the anterior and lateral is not constant in man; the anterior tract is absent in about 15 per cent. of human cords. This is of interest when we recall that in lower animals the tract is undivided. In cats the whole pyramidal tract decussates to the lateral funiculus of the spinal cord while in moles the entire tract descends the an terior funiculus without decussation or division. The pyramidal tract decussates without division to the opposite posterior funiculus of the spinal cord in the albino rat, guinea-pig, mouse, red squirrel and chipmunk (Ranson: Am. Jour. Anat., Vol. 14, and Simpson: Jour. Comp. Neurol., Vol 24).
The dorsal part of the anterior column is occupied by the substantia reticularis alba (Figs. 123 and 124). It contains the bodies of very few nerve cells and is of a light color. The medial accessory olivary nucleus is imbedded in it near the pyramid and among the fibers of the fillet and three definite bundles of longitudinal fibers have been demonstrated in it, namely, the medial fillet, the anterior tecto-spinal tract and the medial longitudinal bundle.
Medial Fillet (lemniscus medialis).—Just dorsal to the pyramid in the anterior area of the medulla at the level of the olives is a large bundle of fibers called the interolivary stratum of the fillet (Figs. 123 and 125). Situated between the inferior olivary nuclei, it is on that account so named. Superiorly, it is continued as the medial fillet. The medial fillet rises from the nucleus funiculi gracilis and nucleus funiculi cuneati of the medulla and crosses through the median raphe in the fillet decussation. As the fillet runs brainward, it receives fibers from the terminal nuclei of common sensory cerebral nerves and from the vestibular nuclei. Along its lateral border it is accompanied for a short distance in the pons by the lateral fillet. The medial fillet is composed of ascending axones from the terminal nuclei of spinal and cerebral nerves. It carries ordi nary sensations (tactile and muscular) to the superior quadri geminal colliculus by the few fibers constituting the superior fillet, and to the thalamus by the greater part of the medial fillet.
The medial longitudinal bundle (fasciculus longitudinalis medians) (Figs. 123 and 126) which we have studied in the mid brain and pons, constitutes a very distinct strand in the superior half of the medulla, but below the level of the olive it can be identified in the medulla oblongata only by a study of its medullation or of its degeneration. It is continuous with the anterior fasciculus proprius of the spinal cord. Its location is next the median raphe and the ventricular gray substance, immediately anterior to the hypoglossal nucleus, in the upper medulla. The same position is occupied by it in the mid-brain
and pons. It is here in the medulla that the hypoglossal fibers are supposed to enter it and run up to the colliculus facialis, where they join the facial nerve at the internal genu. At the middle of the medulla the decussation of the fillet pushes this bundle forward and somewhat away from the median plane, so that it runs between the fillet decussation and the medial accessory olivary nucleus. Below the level of the fillet decus sation it runs between the decussatio pyramidum and the isolated head of the anterior columna of gray substance. Rising primarily in the gray matter of the cord, the ascending part of the medial longitudinal bundle is augmented in the medulla and pons by fibers from the terminal nuclei of sensory cerebral nerves. Most of its ascending fibers cross the median line and terminate in the motor cerebral nuclei on the opposite side; these form the middle links in many reflex arcs; a few decus sate in the posterior commissure and end in the thalamus. The latter are sensory conduction fibers. The descending part of the medial longitudinal bundle is the anterior reticulo-spinal fasciculus (see pp. 152, 235 and 3o1).
The anterior tecto-spinal bundle (Figs. 123 and 126) descends as a distinct strand from the opposite superior colliculus of the corpora quadrigemina to the medulla; there it approaches and mingles with the medial longitudinal bundle; it is continued along the fissural surface of the anterior column in the cord. Its termination is in the central gray substance, chiefly the cilio-spinal centers. It forms the middle link in the visual reflex arc. Its bulbar and spinal portions constitute chiefly the pupillo-dilator tract (see pp. 135, 233 and 303).
Longitudinal Fibers of the Lateral Colunin.—The contents of the lateral column (Figs. 123 and 124) are as follows : Super ficially, the lateral fasciculus proprius, the vestibulo-spinal, and the ventral spino-cerebellar tracts and the spino-thalamic tract. Deeply lies the substantia reticularis grisea. Within it close to the ventral spino-cerebellar fasciculus descend the rubro spinal, thalamo-spinal, lateral tecto-spinal and lateral reticulo spinal 'tracts and the thalamo-olivary fasciculus runs down along the dorsal surface of the inferior olivary nucleus close to the hilus. Imbedded also in the substantia reticularis are the nucleus ambiguus, the nucleus lateralis inferior and the dorsal accessory olivary nucleus, and in the fasciculus proprius is the main inferior olivary nucleus. The gray matter of the substantia reticularis grisea is a part of the disintegrated anterior columna of the cord and, unlike that of the anterior column, it contains the bodies of many large nerve cells.