THE MAMMALIAN NERVOUS SYSTEM 109 



655-660, pp. 844, 845, Fig. 670, p. 849; Piersol ('13), pp. 1223- 

 1225; Quain ('09), Figs. 232, 233, pp. 228, 229, Fig. 243, p. 240. 



137. Auditory system. — The medial geniculate body of the 

 sheep (thalamic auditory center) and the inferior colliculus 

 (midbrain auditory center) should again be located (Fig. 12), 

 also the arm of the inferior colliculus (brachium quadrigeminum 

 inferius), which is the auditory path between the inferior 

 colliculus and the medial geniculate body. 



Auditory projection fibers pass from the medial geniculate 

 body through the internal capsule to the temporal lobe of the 

 cerebral cortex, but these cannot well be separated by dissection 

 in the sheep. 



138. Dissection of the pyramidal tract. — As the last step of the 

 sheep dissection, by careful tearing down of the fibers follow 

 out some of the internal capsule fibers into the regions of the 

 thalamus, midbrain, and medulla oblongata. Try especially 

 to work out the cortico-spinal (pyramidal) tract. Although 

 functionally a motor and therefore a descending tract, it can 

 more easily be traced from the oblongata upward to the higher 

 centers. It appears as an eminence (pyramis) on the ventral 

 surface of the oblongata below the pons near the midline 

 (cf . Section 95) . Its fibers interdigitate with those of the pons, 

 through which the cortico-spinal tract can be traced. It can 

 then be followed along the ventral surface of the mesencephalon 

 through the pedunculus cerebri into the internal capsule. These 

 fibers arise from the cortical neurones of the superior frontal 

 gyrus; see King ('11) and Simpson and King ('11). 



139. Internal capsule. — From the reference books • master 

 the topographic relations of the functional systems of fibers in 

 the internal capsule. For the connections of these tracts in the 

 human brain see the following: Barker ('01), pp. 666-746 and 

 875-1048; Cunningham ('15), Figs. 567-573, pp. 638-643, also 

 account on p. 642; Herrick ('15), Figs. 45, 79-83; Howell ('15), 

 Fig. 82, p. 185; Morris ('14), Figs. 692-700, pp. 880-888; Piersol 

 ('13), Figs. 1009-1012, pp. 1170-1174; Villiger ('12), Figs. 

 128-136, pp. 136-143. 



140. The dissection of the cerebrum of the sheep's brain, as 

 outlined in the preceding sections, can be completely carried 

 out on one lateral half of the brain. The other half of the 



