438 GENERAL ARRANGEMENT OF 



substance, coming up from below, and those which continue upward to 

 the convolutions of the hemispheres. The cerebellum, in the quadru- 

 peds, is somewhat enlarged by the increased development of its lateral 

 portions, and shows an abundance of transverse convolutions. It con- 

 ceals from view the fourth ventricle and the greater part of the medulla 

 oblongata. 



In the more highly developed quadrupeds, the cerebral hemispheres 

 increase in size so as to cover more or less completely the olfactory 

 ganglia in front and the cerebellum behind. Their surface also becomes 

 covered with numerous convolutions, which are mainly longitudinal or 

 curvilinear in direction, instead of being transverse as in the cere- 

 bellum. 



In Man the development of the cerebral hemispheres reaches its 

 highest point, so that they preponderate completely over all the other 

 nervous centres in the cranial cavity. In the human brain, accord- 

 ingly, when viewed from above, there is nothing to be seen but the 

 convex convoluted surface of the hemispheres ; and even in a posterior 

 view they conceal everything but a portion of the cerebellum. The 

 remaining parts, which are concealed by the cerebrum and cerebellum, 

 participate, however, in the structure of the entire encephalon, and 

 form, as in the lower animals, a series of associated nervous centres and 

 connecting tracts of nerve fibres. 



As the spinal cord passes upward into the cranial cavity, it enlarges, 

 by a kind of lateral expansion, to form the medulla oblongata. This 

 portion of the cerebro-spinal axis is distinguished from the cord below, 

 not only by its external form, but also by the somewhat different 

 arrangement of its gray and white substance. The gray substance, 

 which in the cord presents on each side, in front and rear, the pro- 

 jections of the anterior and posterior horns, recedes, in the medulla 

 oblongata, more and more in a backward direction, and becomes accu- 

 mulated in a nearly single mass at its posterior surface. At the same 

 time, the masses of white substance on each side of the posterior median 

 fissure, which in the cord are called the " posterior columns," diverge 

 from each other at an acute angle, leaving between them the space of 

 the fourth ventricle, and assume the name of the resitform bodies. 

 They become continuous with the inferior peduncles of the cerebellum, 

 and send some of their fibres, in a radiating direction, into the white 

 substance of the cerebellum, to terminate in the gray substance of its 

 convolutions. The floor of the fourth ventricle, thus exposed by the 

 divergence of the posterior columns, is formed by the graj 7 substance 

 of the medulla oblongata, which is accordingly continuous with that of 

 the cord, although it has a different position and a different form. 



Yiewed in front, the medulla oblongata presents two longitudinal 

 eminences of white substance, one on each side of the median line, the 

 anterior pyramids, which take the place of the anterior columns of the 

 cord. At their commencement below, the anterior pyramids are narrow, 

 but grow wider as they ascend. At their lower portion they exhibit a 



