THE INTERNAL STRUCTURE OF THE MESENCEPHALON 163 



The Lemnisci. In sections through the rostral border of the pons the two 

 lemnisci form a broad curved band in the ventral and lateral portions of the 

 tegmentum. The fibers of the lateral lemniscus are cut obliquely, indicating 

 that they have begun to turn dorsally toward the inferior colliculus (Fig. 112). 

 On entering the midbrain this lateral portion of the fillet separates from the 

 medial lemniscus and runs toward the corpora quadrigemina, where it forms a 

 capsule for the nucleus of the inferior colliculus (Fig. 114). Some of these 

 fibers are prolonged beyond the nucleus and decussate with similar fibers from 

 the opposite side. A large proportion of the fibers of the lateral lemniscus end 

 in the inferior colliculus, but others form the inferior quadrigeminal brachium 

 (Fig. 114), through which they reach the medial geniculate body (Figs. 116, 134). 

 In the mesencephalon the lateral lemniscus, which, it will be remembered, is the 

 central auditory tract from the cochear nuclei, is joined by the fibers of the 

 spinotectal tract; and these run with it to the corpora quadrigemina. 



The medial lemniscus, or bulbothalamic tract from the gracile and cuneate 

 nuclei of the opposite side, is continued through the tegmentum of the mesen- 

 cephalon to end in the lateral nucleus of the thalamus (Fig. 235) . Incorporated 

 with it in this upper part of its course are the fibers of the spinothalamic tract 

 and a portion of the central sensory tract of the trigeminal nerve (Figs. 132, 234). 

 In the caudal part of the mesencephalon this broad band of longitudinal fibers 

 occupies the ventrolateral portion of the tegmentum (Fig. 114); but at the level 

 of the superior colliculus it has been displaced dorsolaterally by the red nucleus. 

 Here it lies not far from the medial geniculate body and inferior quadrigeminal 

 brachium (Fig. 116). 



The Central Gray Stratum. The cerebral aqueduct is lined by ependymal 

 epithelium and surrounded by a thick layer of gray matter, the central gray 

 stratum, which, because of its paucity in myelinated fibers, is nearly colorless in 

 Weigert preparations. This layer is continuous with the gray matter surround- 

 ing the third ventricle, on the one hand, and with that covering the rhomboid 

 fossa on the other. Numerous nerve-cells of various size and shape are scat- 

 tered through this central gray substance; and, in addition, there are three 

 compact groups of cells, which are the nuclei of the oculomotor and trochlear 

 nerves and of the mesencephalic root of the trigeminus. 



The nucleus of the trochlear nerve contains the cells of origin of the motor 

 fibers for the superior oblique muscle of the eye. It is a small oval mass situated 

 in the ventral part of the central gray stratum at the level of the inferiof collic- 

 ulus (Fig. 1 14) . The fibers of the trochlear nerve emerge from the dorsolateral 



