364 



THE NERVOUS SYSTEM 



52. Carefully examine the medulla, pans, floor of the fourth ventricle, and the mesen- 

 cephalon, observing all the details mentioned on pp. 118-131 and illustrated in Figs. 



84, 86-89, 91. 



53. Take selected transverse sections through the human brain stem and, by com- 

 parison with the gross specimen, determine the level of each section. 



54. Draw in outline each of these transverse sections through the brain stem. 

 Put each drawing on a separate page, ventral side down, with the transverse diameter 

 corresponding to the longer dimension of the paper. Study each preparation in detail 

 and identify all of the parts, indicating them lightly in pencil. Do not label the draw- 

 ings at this time. Make sure that all proportions are correct. The sections through 

 the medulla should be enlarged eight diameters, those through the pons and mesen- 

 cephalon four diameters. 



55. Section Through the Decussation of the Pyramids. Keep in mind the tracts 

 which extend into the brain from the spinal cord and note the changes in their form 

 and position. Identify the decussation of the pyramids, the nucleus gracilis and nucleus 

 cuneatus, the spinal root of the trigeminal nerve and its nucleus, the reticular formation. 

 Note the change in the form of the gray substance (pp. 132-137; Figs. 94, 95, 98). 



56. Section Through the Decussation of the Lemniscus. Note the rapid change in 

 the form of the gray matter. Identify the internal and external arcuate fibers, the 

 decussation of the lemniscus and the beginning of the medial lemniscus, as well as the 

 structures continued up from the preceding level (Figs. 96, 99; pp. 137-139). 



57. Section Through the Olive and the Hypoglossal Nucleus. At this level the central 

 canal opens out into the fourth ventricle. The posterior funiculi and their nuclei are 

 disappearing or have disappeared. The dorsal spinocerebellar tract lies lateral to the 

 spinal tract of the trigeminal nerve and is directed obliquely backward toward the 

 restiform body. Identify, in addition to those structures which are continued from 

 the preceding level, the inferior olivary nucleus with the olivocerebellar fibers, the 

 dorsal and medial accessory olivary nuclei, the external arcuate fibers, the nucleus and 

 fibers of the hypoglossal nerve, the dorsal motor nucleus of the vagus, the tractus 

 solitarius and its nucleus, the nucleus ambiguus and the lateral reticular nucleus (Figs. 

 97,101; pp. 139-142). 



58. Section Through the Restiform Body. The restiform body and the spinal tract 

 of the fifth nerve are conspicuous in the dorsolateral part of the section. In the floor 

 of the fourth ventricle locate the nucleus of the hypoglossal nerve, the dorsal motor 

 nucleus of the vagus, the medial and the spinal vestibular nuclei. The spinal tract of 

 the fifth nerve and its nucleus are deeply situated ventral to the restiform body and 

 broken up by the olivocerebellar fibers (Fig. 103; pp. 143-146). 



59. Section Through the Lower Margin of the Pons. Identify such portions of the 

 pons, brachium pontis, and cerebellum as are contained in the section. Dorsolateral 

 to the restiform body is the dorsal cochlear nucleus, and ventrolateral to it the ventral 

 cochlear nucleus. Identify the striae medullares and the beginning of the trapezoid 

 body, also the medial and lateral vestibular nuclei (Fig. 107; pp. 149-152). ' 



60. Section Through the Facial Colliculus. Differentiate between the ventral and 

 the dorsal portions of the pons, and in the ventral portion identify the longitudinal 

 fasciculi, transverse fibers, and the nuclei pontis (pp. 147-149). In the dorsal part 

 identify the nuclei and root fibers of the sixth and seventh nerves including the genu 



