THE INTERNAL STRUCTURE OF THE MESENCEPHALON 167 



run to and terminate within the medial geniculate body (Figs. 114, 116). The 

 fibers of the lateral lemniscus carry auditory impulses from the terminal nuclei 

 of the cochlear nerve. Some of these terminate in the inferior colliculus and 

 are concerned with reflexes in response to sound. Other fibers, some of which 

 are branches of those to the inferior colliculus, run to the medial geniculate 

 body, from which the impulses that they carry are relayed to the cerebral cor- 

 tex. The inferior quadrigeminal brachium also contains fibers of cortical origin, 

 chiefly from the temporal lobe, which end within the inferior colliculus (Beevor 

 and Horsley, 1902). 



The superior colliculi, or superior quadrigeminal bodies, are composed of 

 laminated gray matter. Each consists of four superimposed, dor sally convex 

 layers (Fig. 116). The most superficial of these is a thin lamina with many 

 transversely coursing nerve-fibers, the stratum zonale. The second layer is much 

 thicker, contains few myelinated fibers, and is known as the stratum griseum. 

 The third and fourth layers, stratum opticum and stratum lemnisci, are rich in 

 myelinated fibers. The majority of the afferent fibers of the superior colliculus 

 come from the optic tract by way of the superior quadrigeminal brachium and 

 enter the stratum opticum. Many of these end in the superimposed stratum 

 griseum. The superior colliculus also receives fibers from the cerebral cortex 

 and from the spinotectal tract. 



It has been generally supposed that the fibers of the stratum zonale come from the 

 optic tract, but according to Cajal (1911) this cannot be the case, since they remain intact 

 in animals which have been operated on in such a way as to produce degeneration of the optic 

 fibers. According to him it is also probable that the fibers from the cerebral cortex, which 

 reach the colliculus by way of the superior quadrigeminal brachium, end in the stratum 

 lemnisci. The fibers of the spinotectal tract run with the lateral lemniscus in the upper part 

 of its course and enter the superior colliculus by way of the stratum profundum. 



The tectobulbar and tectospinal tracts have their origin within the tectum of 

 the mesencephalon, more of the fibers coming from the superior than from the 

 inferior colliculi. These fibers, arising from cells in more superficial layers, are 

 assembled in the stratum profundum and sweep ventrally in broad curves around 

 the central gray substance (Figs. 116, 118). The majority of the fibers, after 

 crossing the median plane in the dorsal tegmental decussation, run in a caudal 

 direction just ventral to the medial longitudinal bundle in the tectospinal tract. 

 They give off collaterals to the reticular formation and the red nucleus. But 

 some of them, instead of taking part in this decussation, leave the mesencephalon 

 by way of the lateral lemniscus of the same side, constituting the lateral tecto- 

 bulbar and tectospinal tracts (Cajal, 1911; Edinger, 1911). 



