GENERAL CONFORMATION AND HISTOLOGY OF BRAIN. 39 



the outer part of the lenticular nucleus (Fig. 21, hclow and to 

 the right). 



I cannot urge you too strongly, gentlemen, to look up in 

 the fresh brain all the parts mentioned in to-day's lecture, and 

 learn the situation of each by your own dissection. The repre- 

 sentation by word and picture may give you a- good idea of 

 them, but it can never take the place of that which may be 

 obtained by study of the fresh specimen. 



The difference in colors which you have observed to-day in 

 the fresh brain specimens is caused by differences in their finer 

 structure. To this, the histology of the central nervous system, 

 we will now, for a short time, turn our 

 attention. 



The brain is made up of nerve- 

 tissue and a matrix. This matrix is 

 composed of the walls of numerous 

 blood-vessels which pass in every direc- 

 tion through the organ, and of the 

 neuroglia, most delicate cells, with a 

 net-work of fine ciliary processes, which, FIG. 22. 



i , i i , ,1 ft Isolated neuroelia-cells. 



becoming intermingled with those 01 



neighboring cells, are matted together into a sort of dense felt. 

 This structure may well be likened to a mass of burrs stuck 

 together with a few thicker strands, the blood-vessels, passing 

 among them. The nerve-fibres are imbedded in the free spaces 

 between the cells of the neuroglia. 



The net-work of neuroglia is somewhat differently con- 

 stituted in different parts of the central nervous system, and 

 forms here and there thick masses, altogether free from nerve- 

 substance. 



Thus, for instance, a broad zone of almost unmixed neu- 

 roglia is spread over the whole surface of the brain and spinal 

 cord. The larger ganglion-cells are often so entangled in neu- 

 roglia that they seem to be lying in a fine-meshed basket. 



