192 PRACTICAL MICROSCOPY. 



medullated, but have no neurilemma. The neuroglia and ganglionic 

 tissues do not here differ in structure from that previously de- 

 scribed. The gray substance is arranged in layers, which are, in 

 .some instances, quite sharply defined, and again demonstrable only 

 with considerable difficulty. 



PRACTICAL DEMONSTRATION. 



The tissue is to be prepared in the manner usual with nerve sub- 

 stance hardened with Miiller, followed by alcohol. Thin sections, 

 stained deeply with haema. and eosin, may be mounted in dammar or, 

 if preferred, in glycerin. 



SECTION OF HUMAN CEREBRUM. CUT PERPENDICU- 

 LARLY TO THE SURFACE. (Fig. 124.) 



OBSERVE: 



1. The membranes. (In the drawing only the arachnoid and pia 

 are shown.) (a) The fine fibrillar mesh of the arachnoid. 

 (b) The nuclei of the flattened cell-covering, (c) The large 

 blood-vessels, (d) The pia. (e) Its continuity with the neuro- 

 glia of the cerebrum. 



2. The outer layer the first of the gray substance. (This layer 

 is poorly defined, but can usually be made out. It consists of primitive 

 nerve fibrillae, neuroglia fibrils, and scattered spherical cells.) 



3. The second layer. (This layer presents about the same thick- 

 ness as the preceding, and will be recognized by the numerous small, 

 triangular nerve-cells. Indeed, these afford the only means of dis- 

 tinguishing the boundary between the two layers, as the stained ele- 

 ments of the outer layer are seldom pyramidal.) 



4. The third layer. (This layer the thickest of all the gray 

 laminae has been shortened in the drawing, on account of lack of 

 space. It is three or four times as thick as the first layer, and will be 

 readily made out by the large, elongate, pyramidal cells, with their 

 long axes at right angles to the brain surface. Medullated fibres, in 

 more or less distinct bundles, pass between the column-like ganglion 

 cells.) 



5. The fourth layer. (The large cells of the third layer are seen 

 to stop abruptly, as we pass inward, and give place to small, triangular 

 nerve-cells. This brings us to the fourth plane. Between the cells of 

 this layer, bundles of nerve-fibres are seen, as they radiate toward the 

 cerebral surface.) 



