THE MAMMALIAN NERVOUS SYSTEM 51 



relation of a special part of the olfactory nerve to the vomero- 

 nasal organ (Jacobson's organ) see McCotter ('12) and ('17). 



48. Brain stem and cortex. Observe the relations of the 

 cerebral cortex (which makes up the greater part of the cere- 

 bral hemispheres) and the cerebellum. With a scalpel cut off a 

 slice about 1 cm. thick from the posterior pole of the left cere- 

 bral hemisphere and a similar slice from the left lateral border 

 of the cerebellum. Compare the cut surfaces and observe in 

 each the relations of the superficial gray matter (cortex) to 

 the underlying white matter. The cerebral cortex and cere- 

 bellum make up the suprasegmental apparatus, as distin- 

 guished from the spinal cord and brain stem, or segmental 

 apparatus; see Herrick ('18), Chap. VII, and A. Meyer ('98), 

 pp. 136-147. 



The isthmus is a constriction of the brain in front of the 

 medulla oblongata and cerebellum. It divides the brain into 

 rhombencephalon and cerebrum. The rhombencephalon wil be 

 examined before the cerebrum. Each of these subdivisions 

 consists of a basal or segmental part and a suprasegmental part 

 (cerebellum and cerebral cortex respectively). The supraseg- 

 mental apparatus overlaps the brain base, or brain stem, whose 

 functions it correlates. 



49. The rhombencephalon. In both the human and the 

 sheep's brains observe the mode of attachment of the cere- 

 bellum to the medulla oblongata, or bulb. With a scalpel cut 

 these attachments (cerebellar peduncles) on each side, remove 

 the cerebellum, and lay it aside for future study, performing 

 this operation first on the sheep's brain,. then on the human. 

 These peduncles should be severed as high up as possible, 

 cutting into the substance of the cerebellum somewhat rather 

 than into the substance of the medulla oblongata. In making 

 this dissection be careful not to injure the delicate membranes 

 lying below the cerebellum and forming the roof of the fourth 

 ventricle. This can readily be accomplished in the sheep's 

 brain. Unless the human brain is well preserved, these mem- 

 branes may be destroyed in this case. 



50. The cavity of the rhombencephalon is the fourth ven- 

 tricle (ventriculus quartus or fossa rhomboidea). The cere- 

 bellum itself forms the roof of this ventricle for a short 



