The superior colliculi of the corpora quadrigemina (Figs. 58 and 63) represent the optic lobes of birds, fishes and reptiles. They contain the center of optic reflexes. In being stratified, they bear some resemblance to the lateral geniculate bodies. They possess three white and two gray layers: (r) The stratum zonale (stratum album superficiale) is a layer of white matter on the surface. This invests (2) the laminated stratum griseum, which forms the deep part of the colliculus and lies upon (3) the stratum album profundum. The stratum griseum superficiale is composed of small mutlipolar cells. The stratum album medium is a layer of fibers separating the small from the large multipolar cells. The large cells make up the stratum griseum profundum, underneath which is the deep layer of fibers, the stratum album profundum.' The fibers of the superficial, middle and deep strata comprise, first, those that enter the colliculus through the brachium superius, through the superior and a part of the lateral fillet and the spino-thalamic tract; and, second, those that take origin in the colliculus and leave it through the brach ium superius or the tecto-spinal fasciculi and other descending bundles. Of the fibers originating in the superior colliculus and running through the brachium superius it is supposed that some go as far as the retina; probably others enter the cortical fillet. The descending axones of the superior colliculus joined by a smaller number from the inferior colliculus, constitute the tecto spinal and the tecto-cerebellar fasciculi and a part of the tecto pontal and tecto-reticular fasciculi which rise chiefly in the infe rior colliculus.
The tecto-cerebellar fasciculi enter the cerebellum through the superior medullary velum; they bring the cerebellum into the olfactory, visual and acustic (cochlear) correlations.
The tecto-spinal fasciculi are two in number, an anterior and a lateral. The short fibers of these tracts end in the cranial nerve nuclei and form the tecto-bulbar fasciculi. Formerly the anterior tecto-spinal fasciculus was called the anterior longitudinal bundle. The anterior tecto-spinal fasciculus is made up chiefly of efferent axones of the cell-bodies in the superior colliculus. It crosses at once through the dorsal tegmental decussation (Fig. 63) and descends ventro-lateral to the opposite medial longi tudinal bundle, to the anterior columna of gray matter in the spinal cord. Its fibers end largely in the nuclei of the third, fourth and sixth cerebral nerves and in the cervical enlargement of the spinal cord; but perhaps others enter the remaining nuclei of motor cerebral nerves, and a few fibers of the tract have been traced as low as the lumbar region. This bundle is the great optic reflex tract. The fibers to the nuclei of the third, fourth and sixth cerebral nerves bring about the reflex movements of the eyeball, contraction of the pupil and accommodation to distance; while those fibers which end in the gray substance of the lower part of the cervical enlargement of the spinal cord, called the cilio-spinal center, through the white rami communi cantes and cervical sympathetic, produce dilatation of the pupil. The latter constitute the pupillo-dilator tract.
The lateral tecto-spinal fasciculus has the same origin as the anterior fasciculus; but it is largely if not wholly uncrossed. It descends through the lateral part of the reticular formation and the lateral column of the spinal cord in close relation with the thalamo-spinal and rubro-spinal bundles. The fibers of the
lateral tecto-spinal fasciculus end in the gray matter of the brain stem and spinal cord.
Destructive lesions affecting the superior quadrigeminal col liculi produce loss of reflex movement of the eyeballs, loss of pupillary reflex and loss of accommodation.
The inferior colliculi of the corpora quadrigemina form a relay in the auditory path (Figs. 62 and 64). They are made up of a white stratum zonale, whose fibers are continuous chiefly with the lateral fillet and brachium inferius, and of a deep gray mass, the nucleus colliculi inferioris, which is com posed of small multipolar cell-bodies in a network of fibers. The nucleus rests upon the stratum album profundum. The nuclei of the two eminences fuse in the median plane. In the nuclei end a considerable number of fibers belonging to both lateral fillets, but most of them belong to that of the same side; and from them proceed axones of the auditory paths through the brachia inferiora to the medial geniculate bodies. At this level the acustic path becomes entirely crossed. A few fibers of the spino-thalamic tract also end in the inferior colliculus. Again this colliculus receives corticifugal fibers of the temporo thalamic radiation.
Axones from the nucleus of the inferior colliculus either run forward through the brachium inferius to the medial geniculate body, with the lateral fillet forming a segment of the acustic path, or they descend in the tecto-spinal, tecto-cerebellar, tecto pontal and tecto-reticular fasciculi. The first two, which rise chiefly in the superior colliculus, have been described.
The tecto-pontal fasciculus (of Miinzer) rises largely in the inferior colliculus and ends within the nucleus pontis.
The tecto-reticular fasciculus (of Pawlow) is a small bundle rising in the quadrigeminal region and, as its name indicates, terminating in the nuclei of the reticular formation, chiefly in the pons.
Though the greater part of the lateral fillet passes by the inferior colliculus without relay, a lesion in this body is apt to involve the entire bundle and cause almost complete deafness in the opposite ear.
Nucelus Lateralis Superior (Fig. 63).—In the reticular for mation of the tegmentum at the level of the superior quad rigeminal colliculus is the nucleus lateralis superior. It con tains large multipolar cell-bodies and, being imbedded deeply in the tegmentum it is properly called nucleus tegmenti profundus (see pp. i5i and 155). The nucleus forms a relay both for ascending and descending paths of the formatio reticu laris. According to Tschermak, a small fasciculus runs from this nucleus into the medial longitudinal bundle where it divides T-like; and its descending,fibers run down through the anterior fasciculus proprius of the cord (Barker). This is the anterior reticulo-spinal fasciculus, an uncrossed tract. Other axones of the nucleus tegmenti profundus decussate in the mid brain and descend as lateral reticulo-spinal fasciculus described on p. 155.
Substantia Nigra (Figs. 61, 62, 63 and 64).--The small pig mented multipolar cell-bodies which make up the substantia nigra form, first, a terminal nucleus for certain fibers of the medial fillet and a nucleus of origin for other fibers which con tinue in that tract (Barker); and, second, a terminal station for the fasciculus habenulo-peduncularis, or retroflexus (Mey nerti) and a relay between the globus pallidus and nucleus pontis.