566 THE NERVOUS SYSTEM 



rest of the brain. At the same time the smooth surface of the cerebral 

 cortex of many lower mammalia, such as the rabbit, is replaced by the laby- 

 rinth of convolutions of the human brain. 



When the cerebral hemispheres are removed, several large basal masses 

 of nerve substance are revealed: the optic thalami, the corpora quadrigemina, 

 and the cms cerebri. These structures, together with the pons and the me- 

 dulla, form a direct continuation forward of the spinal cord and sometimes 

 are designated under the general term of the brain stem. 



FIG. 375. Plan in Outline of the Brain as seen from the Right Side. X . The parts 

 are represented as separated from one another somewhat more than natural, so as to show 

 their connections. A, Cerebrum; /, g, h, its anterior, middle, and posterior lobes; e, 

 fissure of Sylvius; B, cerebellum; C, pons Varolii; D, medulla oblongata; a, peduncles of the 

 cerebrum; b, c, d, superior, middle, and inferior peduncles of the cerebellum. (From 

 Quain.) 



The human brain on superficial examination does not seem to follow the 

 general plan outlined above, but when the cerebral hemispheres and the 

 cerebellum are removed then it is found that what remains closely follows 

 the plan presented. This central axis is shown in part in figure 379. 



The morphological parts of the brain usually given are: 



The Brain or Encephalon. 



I. The hind-brain or rhombencephalon. 



1. Myelencephalon. 



a. Bulb or medulla oblongata. 



2. Metencephalon. 



b. Pons Varolii. 



c. Cerebellum. 



