THE NERVOUS SYSTEM 199 



The pia mater is a very vascular membrane. It in- 

 vests nearly the entire surface of the brain, and dips in 

 between its convolutions. It is the nutrient membrane of 

 the brain, and receives all the blood from the carotid and 

 vertebral arteries. 



The Cerebrum. The cerebrum is divided into two 

 hemispheres by a cleft or fissure. In the fissure we find 

 a circular or arched projection of the dura mater, called 

 the falx cerebri. The object of this construction is to 

 render support to this large pulpy mass. The upper 

 surface of the cerebrum is marked by many undulations 

 or convolutions, and the under surface admits of three 

 divisions in each hemisphere the anterior, the middle, 

 and the posterior divisions. 



When the brain is cut through its upper part hori- 

 zontally, and the part removed so as to expose the cut 

 surface, it will be discovered that the substance of this 

 organ is of two distinct colors. On its outer, surf ace the 

 brain, for a small depth, is of an ashen-gray or cineritious 

 color, while the central portion is white. This darker 

 portion is called its cortical or cineritious portion; in its 

 outlines it follows the convolutions of the brain. The 

 light-colored portion is called the ^medullary portion. 

 The two hemispheres of the cerebrum are connected by a 

 dense layer of transverse fibers, called the corpus callosum. 



The Cerebellum. This portion of the brain is situated 

 in the occipital or back part of the skull. Like the sub- 

 stance of the brain, the substance of this portion is also of 

 two colors. When the cerebellum is cut vertically, the 

 white matter has a beautiful appearance, resembling the 

 trunk and branches of a tree, and known as the "arbor 

 vita" The main stem of this structure contains a col- 

 lection of gray matter, the corpus dentatum. 



The Medulla Oblongata. By referring to the illustra- 

 tion (Fig. 102) you will see, at the base of the brain, the 



