THE MID-BRAIN OR MESENCEPHALON 



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FIG. 714. Transverse section passing through 

 the sensory decussation. Schematic. (Testut.) 

 1. Anterior median fissure. 2. Posterior median 

 sulcus. 3, 3'. Head and base of anterior column 

 (in red). 4. Hypoglossal nerve. 5. Bases of 

 posterior column. 6. Gracile nucleus. 7. Cune- 

 ate nucleus. 8, 8. Lemniscus. 9. Sensory 

 decussation. 10. Cerebrospinal fasciculus. 



The central or thalamic tract of the cranial nerves is closely associated with the 

 medial lemniscus. The fibers of the spinothalamic fasciculi are continued from the 

 spinal cord into this tract which passes upward in the reticular formation and the 

 tegmentum to the thalamus along the dorsal side of the median lemniscus. It 

 receives fibers from the opposite terminal sensory nuclei of the vagus, glossopharyn- 

 geal, facial, trigeminal and probably the vestibular nerves. Many of the secondary 

 sensory fibers of the trigeminal cross the raphe from its terminal nucleus and pass 

 upward to the thalamus by a more or less separate but closely associated pathway 

 known as the central tract of the trigeminal nerve 

 which also lies on the dorsal aspect of the lemnis- 

 cus. These two tracts give off collaterals to the 

 posterior semilunar nucleus of the thalamus and 

 terminate in the anterior semilunar nucleus of 

 the ventro-lateral region of the thalamus sending 

 collaterals into the zona incerta. 



The fibers of the rubrospinal tract (bundle of 

 Monak&w) arise in the red nucleus, cross the 

 midline in the decussation of Forel and pass 

 downward in the f ormatio reticularis of the brain- 

 stem into the lateral funiculus of the spinal cord 

 ventral to the crossed pyramidal tract. 



The lateral lemniscus (lemniscus lateralis} 

 comes to the surface of the mid-brain along 

 its lateral sulcus, and disappears under the 

 inferior colliculus. It consists of fibers from 

 the terminal nuclei of the cochlear division 

 of the acoustic nerve, together with others from 

 the superior olivary and trapezoid nuclei. Most 



of these fibers are crossed, but some are uncrossed. Many of them pass to the 

 inferior colliculus of the same or opposite side, but others ^ are prolonged to 

 the thalamus, and thence through the occipital part of the internal capsule to 

 the middle and superior temporal gyri. 



The corpora quadrigemina (Fig. 720) are four rounded eminences which form 

 the dorsal part of the mid-brain. They are situated above and in front of 

 the anterior medullary velum and superior peduncle, and below and behind the 

 third ventricle and posterior commissure. They are covered by the splenium of the 

 corpus callosum, and are partly overlapped on either side by the medial angle, 

 or pulvinar, of the posterior end of the thalamus; on the lateral aspect, under 

 cover of the pulvinar, is an oval eminence, named the medial geniculate body. 

 The corpora quadrigemina are arranged in pairs (superior and inferior colliculi), 

 and are separated from one another by a crucial sulcus. The longitudinal part 

 of this sulcus expands superiorly to form a slight depression which supports the 

 pineal body, a cone-like structure which projects backward from the thalam- 

 encephalon and partly obscures the superior colliculi. From the inferior end of 

 the longitudinal sulcus, a white band, termed the frenulum veli, is prolonged down- 

 ward to the anterior medullary velum; on either side of this band the trochlear 

 nerve emerges, and passes forward on the lateral aspect of the cerebral peduncle 

 to reach the base of the brain. The superior colliculi are larger and darker in color 

 than the inferior, and are oval in shape. The inferior colliculi are hemispherical, 

 and somewhat more prominent than the superior. The superior colliculi are 

 associated with the sense of sight, the inferior with that of hearing. 



From the lateral aspect of each colliculus a white band, termed the brachium, 

 is prolonged upward and forward. The superior brachium extends lateralward 



Krom the superior colliculus, and, passing between the pulvinar and medial genicu- 



