ANATOMY AND I'llYMOLOGY OF THE BRAIN 359 



white matter in the centre. The gray substance 

 consists of masses of nerve cells, their axones, and 

 dendrites. The cells are arranged in layers. The white 

 matter consists of nerve fibers which pass in different 

 directions and connect various portions of the cere- 

 bellum with one another. Nerve fibers are grouped 

 in bundles and connect cerebellum with the cerebrum, 

 pons varolii, medulla oblongata, and spinal cord. 



The Function of the Cerebellum. It is the centre 

 for maintaining the equilibrium of the body, by 

 sending out nerve impulses, which cause a combined 

 action of groups of muscles that enable the body to 

 stand erect without swaying, and assist in the various 

 and complex movements seen in walking, dancing, 

 running, etc. The centres in the cerebellum are 

 reflexly influenced by nerve impulses arising in the 

 end-organs of the skin, retina of the eye, tactile (touch) 

 sense, and the labyrinth of the ear. These impulses 

 are transmitted to the cerebellum by afferent nerve 

 fibers and they stimulate the centres to activity and 

 the nerve impulses are conveyed by efferent nerves, 

 though the pons, medulla, and spinal cord and nerves 

 to the general muscle system. 



The Cerebrum. The cerebrum is the largest part of 

 the brain, and consists of two lateral halves or hemi- 

 spheres, separated by the great longitudinal fissure 

 and connected to each other by a great commissure, 

 the corpus callosum. The latter constitutes a great 

 system of association nerve fibers for the bilateral 

 coordination of corresponding parts of the nerve cells 

 in the cortex. The hemispheres are subdivided into 

 lobes, and the latter present over their entire surfaces 

 convoluted eminences, the gyri or convolutions, sepa- 

 rated by depressions, the sulci or fissures. (See Figs. 

 126 and 127.) 



The cerebrum, as a whole, is convex from before 

 backward and from side to side, narrower in front 



