CHAPTER XXXVI. 



THE BRAIN. 



As we pass upward in attempting to trace the destiny of the 

 conducting channels of the medulla, we come to the more elabo- 

 rate system of nervous textures which, together, are called the 

 brain. This is anatomically the most highly developed, and 

 physiologically the most intricate part of the central nervous 

 organs. Besides the nerve cells and various kinds of conducting 

 channels with which we have already become familiar in the 

 cord, etc., there are in the brain a vast number of smaller elements 

 which do not possess the distinctive characters of cells. These 

 granular bodies are tightly packed together in many parts of the 

 centres, and must have some important function, which is, how- 

 ever, at present unknown. 



The best way to get an idea of the general plan of construc- 

 tion of the brain, is to follow its development in the earlier stages 

 of the embryo, from the time when it forms an irregular and 

 thickened part of the tube of tissue destined to become the spinal 

 cord. From this it will be seen that the brain is but a modified 

 part of the primitive nervous axis, in which certain swellings may 

 be observed at an early period of embryonic life. These swell- 

 ings are called the fore-brain, the mid-brain and the hind-brain, 

 and in the future development of the parts give rise to (1) the 

 hemispheres and basal ganglia; (2) the corpora quadrigemina, 

 pons and cerebellum ; and (3) the medulla oblongata. The 

 great mass of the brain the hemispheres are formed by an 

 excessive development of bud-like processes which grow out from 

 the sides of the fore-brain at an early period, which become 

 elaborately folded, so that in the adult it is difficult to trace the 

 relationship to the original form. 



The cells of the brain are, like those in the cord, grouped 

 together in the complex gray substance, while the white part is 

 made up exclusively of conducting fibres. The gray substance 



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