THE MAMMALIAN NERVOUS SYSTEM 53 



V tract and its nucleus. These three eminences contain centers 

 of general cutaneous and deep sensibility for the trunk, limbs, 

 and head; that is, in the aggregate they form the general somatic 

 sensory column. 



57. The area acustica. — This is the special somatic sensory 

 column. It comprises the vestibular nucleus in the floor of the 

 fourth ventricle and the cochlear nucleus, a compact crescent- 

 shaped mass of gray matter which encircles the restif orm body 

 at the point where the latter turns dorsalward to enter the cere- 

 bellum (Fig. 11). 



58. Now get from the instructor a long, thin brain knife. 

 Special knives for this purpose are sold by the manufacturers 

 of surgical instruments, but a butcher's ham-slicer, to be pur- 

 chased in the hardware trade, makes a very satisfactory substi- 

 tute. This is a large butcher knife with a long and very thin, 

 wide blade. Better still is a large steel spatula, or "pill knife," 

 such as druggists use, with both edges ground sharp. With 

 this knife cut the entire brain of the sheep into right and left 

 halves. The incision should pass through the longitudinal 

 fissure between the cerebral hemispheres and should cut through 

 the corpus callosum in the floor of this fissure, and then down- 

 ward through the entire brain stem. Great care should be 

 taken to make this cut smooth and exactly in the median plane. 

 It should be made with a single long sweep of the knife. 



Up to this point two students have examined one human 

 brain. Now repeat on the human brain the division into right 

 and left halves in the same way as in the case of the sheep. 

 Divide also the cerebellum by a median incision. Each student 

 takes one-half of the divided human brain. 



59. Examine carefully the cut surfaces of both the human and 

 the sheep's brains, identifying all median structures thus brought 

 into view. Note again the arrangements of the chief subdivis- 

 ions of the brain referred to in Section 38. For the sheep's 

 brain, see Fig. 10. Similar views of the human brain are pic- 

 tured by Cunningham ('15), Fig. 477, p. 542; Quain ('09), Fig. 

 132, p. Ill; Rauber-Kopsch ('07), Fig. 525, p. 443, Fig. 527, 

 p. 447; Spalteholz ('09), Figs. 694, 695, pp. 625, 626; Toldt ('04), 

 Figs. 1193-1195, pp. 776, 777. 



60. Draw the median surface of the human brain (or of the 



