THE MAMMALIAN NERVOUS SYSTEM 49 



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 cerebellum make up the 

 suprasegmental apparatus, as distinguished from the spinal 

 cord and brain stem, or segmental apparatus; see Herrick ('15), 

 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 will 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 cerebellum to the 

 medulla oblongata, or bulb. With a scalpel cut these attach- 

 ments (cerebellar peduncles) on each side, remove the cere- 

 bellum, and lay it aside for future study, performing this opera- 

 tion first on the sheep's brain, then on the human. These pe- 

 duncles should be severed as high up as possible, cutting into the 

 substance of the cerebellum somewhat rather than into the sub- 

 stance 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 membranes may be de- 

 stroyed in this case. 



50. The cavity of the rhombencephalon is the fourth ventricle 

 (ventriculus quartus or fossa rhomboidea). The cerebellum itself 

 forms the roof of this ventricle for a short distance between the 

 cerebellar peduncles. In front of this level in the isthmus region 

 the roof is formed by a thin sheet of nervous tissue, the anterior 

 medullary velum (velum medullar -e anterius) . Behind this level 

 the roof of the fourth ventricle [tegmen fossce rhomboidece) is a 

 thin non-nervous membrane, a part of which is highly vas- 

 cular and much folded; this is the chorioid plexus of the fourth 

 ventricle. If this membrane is intact, carefully pick it up with 



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