A STUDY OF THE NERVOUS SYSTEM 275 



system last mentioned resemble each other more or less in 

 their structure, we will consider the hindbrain first. 



Hindbrain. Examining the brain from the side, we see 

 that the relative position of medulla and cerebellum is the 

 same as in the frog's brain. The human medulla looks like 

 an enlarged portion of the spinal cord, and this was also 

 true of the amphibian brain. The first striking contrast in 

 the appearance of the two brains is the large size of the 

 human cerebellum. In the hindbrain of man, too, a new 

 structure appears in the shape of broad bands of nerve 

 tissue that pass around the ventral surface of the medulla, 

 connecting the two halves of the cerebellum. It is called 

 the pons Va-ro'li-i (Latin pons = bridge + Varolii = of Varo- 

 lius, so named from its discoverer), or more commonly simply 

 the pons (see Figs. 128 and 130). 



Forebrain. The cerebral hemispheres of the forebrain 

 are enormously developed; indeed, they constitute about 

 three fourths of the human brain. They fill the largest 

 part of the cranial cavity, completely envelop the midbrain, 

 and partially cover the hindbrain. A deep fissure sepa- 

 rates the two hemispheres, and at the bottom of this fissure 

 a broad band of white fibers runs across like a bridge from 

 one half of the brain to the other. 



The surface of each hemisphere is raised in ridges ; these 

 are called con-vo-lu'tions (Latin con-vol've-re = to roll up). 

 The various convolutions and the fissures that separate them 

 have been named from adjacent bones of the cranium or from 

 the investigators who have studied them. The most promi- 

 nent groove is the fissure of SyVvi-us, seen at the side 

 of the brain, which divides the upper and lower portions 

 of the cerebral hemispheres. The fissure of Ro-lan'do 

 divides the frontal and parietal lobes. Beneath the an- 

 terior end of each half of the forebrain is a small olfactory 

 lobe (Fig. 128, I). 



Midbrain. Little need be said of the midbrain of man 

 except that it forms an isthmus connecting the fore- and 



