STRUCTURE OF BRAIN. 257 



Trace the blood tubes of the brain. For this the injected 

 brain of a cat or dog should be used. 



Cut and examine the cross sections of the spinal cord after 

 it has been hardened in alcohol. Compare the colors of the 

 inside of the brain and spinal cord. 



It will be observed that the brain, like the spinal cord, 

 consists of two lateral halves. Cutting sections of the brain 

 lengthwise and crosswise shows that the outer part is made 

 up of gray matter and the inner part of white matter. The 

 gray matter is composed of cells essentially similar to those 

 of the spinal cord, while the white 

 matter of the inner part is composed 

 of white fibers like those of the outer 

 part of the spinal cord, or like the 

 nerves. 



In comparing the brain of the rab- 

 bit and the cat, that of the rabbit is 

 found to have fewer convolutions, and Fig. so. 



is nearly smooth. In general, the Pyramidal Nerve Ceils, found 



t principally in the Gray 



lower animals have fewer coiivolu- Matter of the Brain. 



tioiis, and the lower races of man- 

 kind have smoother brains than the higher races. In the 

 earlier stages of development man's brain is smoother; but 

 with growth the convolutions appear, and increase in number 

 with the growth of the brain. As we know that intelligent 

 action depends on the gray matter of the surface of the brain, 

 we infer that to accommodate its increase in the brain-case it 

 is thrown into folds, as the surface of the lining of the intes- 

 tines is increased by folds, villi, etc. 



The gray matter of the convolutions of the adult human 

 brain is about one-fifth of an inch thick, the larger part of 

 the brain consisting of the white matter. Sections will show 

 that there are several masses of gray matter in the brain 



