THE MAMMALIAN NERVOUS SYSTEM 55 



spinal cord where it joins the medulla oblongata will reveal on 

 the dorsal surface the fasciculus gracilis and the fasciculus 

 cuneatus. These are composed chiefly of fiber tracts of the 

 spinal proprioceptive system and they enter enlargements lying 

 laterally of the lower end of the fourth ventricle (calamus 

 scriptorius), known respectively as the clava, or nucleus of the 

 fasciculus gracilis, and the tuberculum cuneatum, or nucleus of 

 the fasciculus cuneatus. Laterally of the tuberculum cunea- 

 tum is the tuberculum cinereum, a longitudinal ridge formed by 

 the spinal 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 resti- 

 form body at the point where the latter turns dorsalward to 

 enter the cerebellum (Fig. 11). The dorsal part of the 

 cochlear nucleus is termed tuberculum acusticum. 



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 

 downward 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. 



