302 



ANATOMY AND PHYSIOLOGY 



projecting arm (laterally and downward). The extremities of the 

 ventricle are called horns; the anterior horn being in the frontal 

 lobe, the posterior horn in the occipital, and the lateral or descend- 

 ing horn in the temporal lobe. The great ganglia of the brain are 

 in the floor of the lateral ventricles (hence called basal ganglia). 



The lateral ventricles are named 

 like the hemispheres right and left. 



(There are certain other basal ganglia 

 which are important, although smaller in 



size.) 



The cerebellum, or little brain, 

 also consists of white matter covered 

 with gray. It has two hemispheres 

 which are not definitely separated 

 like the hemispheres of the cerebrum, 

 but are connected by a median por- 

 tion called the vermis, or worm. The 

 convolutions are but slightly curved 

 and are called ridges, and the furrows 

 (or sulci) are very deep; a section 

 shows that they are so arranged as 

 to resemble the branches of the tree 

 called arbor vita (Fig. 193). 



The cerebellum is situated in the 

 cerebellar fossae of the occipital bone. 



The medulla oblongata, although 



FIG. 194. PONS AND MEDULLA, 

 ANTERIOR SURFACE. 



i, 2, 3, Structures belonging to 

 cerebrum; 4, crura of cerebrum; 5, 

 pons Varolii; 9, 10, n, 13, 14, 

 lateral surface and membranes of 

 medulla; 7, pyramid; 8, decussa- 



nerves; 28, 29, ist and 2 d spinal o f the foramen magnum), is the up- 



nerves. (Sappey.) . , ,. - ,, , 



per enlarged portion of the spinal 

 cord and, like it, is white externally and gray within. 



Its anterior columns are called the pyramids or pyramidal 

 tracts, and consist of motor fibers passing downward from the 

 brain. Most of the fibers of each pyramid cross to the opposite 

 side, appearing to interlace in the median fissure, the decussation 

 of the pyramids, to form the crossed pyramidal tract; the others 

 pass downward as the direct pyramidal tract and cross, a few 

 at a time, at lower levels in the cord. Thus it is that motor 

 fibers coming from one side of the brain pass to the other side 



