THE MAMMALIAN NERVOUS SYSTEM 95 



which lies ventrally of it and the fasciculus longitudinalis 

 medialis and tecto-spinal tract which lie dorsally of it. 



In the midbrain the medial lemniscus fibers split off from 

 those of the lateral lemniscus shortly before the latter enter 

 the inferior colliculus and the medial geniculate body (Herrick 

 ('15), Fig. 75). The medial lemniscus continues almost directly 

 forward, lying ventrally and medially of the lateral lemniscus, 

 and ends in the lateral and ventral nuclei of the thalmus (Her- 

 rick ('15), Figs. 77 and 78), but at this stage of the work it 

 should not be dissected farther forward than the midbrain. 



109. The fasciculus longitudinalis medialis. — In the medulla 

 oblongata the fibers of the fasciculus longitudinalis medialis 

 (posterior longitudinal bundle) will be found running close to the 

 median plane and immediately under the floor of the ventricle. 

 In the lower parts of the medulla this tract lies directly dorsally 

 of the medial lemniscus and separated from it by the tecto- 

 spinal tract, but in the pons region and midbrain these two tracts 

 are far separated. In the dissection of this tract work from the 

 cut median surface lateralward and do not disturb the floor of 

 the fourth ventricle (cf. Sections 92 and 93). 



110. The fasciculus solitarius. — The fasciculus solitarius can 

 now be dissected out. Locate again the ala cinera (trigonum 

 vagi) in the floor of the fourth ventricle. This marks the posi- 

 tion of the dorsal vagal nuclei. Tease off these superficial gray 

 masses and expose the slender fasciculus solitarius which lies 

 below them. Follow this tract downward and upward to its 

 ends (cf. Section 84). 



111. .The pyramidal tract. — Now complete the dissection of 

 the pyramidal tract (tractus cortico-spinalis) from the pons down- 

 ward and note the decussating fibers of this tract on the cut 

 median surface at the lower end of the oblongata. These 

 crossed fibers form the lateral cortico-spinal tract of the cord. 

 A small part of the pyramidal tract does not decussate, but 

 descends directly and forms the ventral cortico-spinal tract of the 

 cord (cf. Sections 94 and 95). 



10. The Cerebrum 



112. The cerebrum comprises all parts of the brain in front of 

 the isthmus. It is further subdivided into the mesencephalon, 



