NERVE-CELLS. 435 



The cell-membrane may be demonstrated easily in the cells of the 

 ganglia, but with great difficulty in those of the central organs, and 

 not at all in the smallest cells of the latter. No membrane can be 

 detected in the processes of the central cells generally. 



The contents of the nerve-cells are a soft, tenacious, elastic, sub- 

 stance, consisting of two elements, besides the nucleus ; (1), a clear, 

 light-yellowish, or colorless, homogeneous substance, an albumin- 

 ous compound very much resembling the axis-fibres, and on which 

 the physical properties of the contents depend; and (2), minute 

 granules of different kinds suspended in the former. These are 

 sometimes larger and yellowish, brown, or blackish ; in which case 

 the cells are colored (as forming the gray matter of the nervous 

 centres) ; while in the colorless nerve-cells they are minute and 

 pale, uniform, and roundish in form. They consist, in great part, 

 of fat. 



The nucleus lies in the midst of the contents just mentioned ; 

 usually as a very clear spherical vesicle with distinct walls, trans- 

 parent contents, and one, and rarely several, opaque nucleoli, occa- 

 sionally exhibiting a cavity. 



Distribution of the Nerve- Cells. 



The nerve-cells occur in great numbers in the gray substance of 

 the encephalon and spinal cord, in the ganglia, and occasionally 

 also in the trunks and peripheral expansions of the nerves ; as in 

 the retina, cochlear nerve, &c. 



Chemical Composition and Physical Properties of the Nerve- Cells. 



The chemical composition of the gray portion of the brain 

 (made up, in great part, of the cells in question), will be given fur- 

 ther on in this chapter. 



If isolated nerve-cells are compressed, they become much flat- 

 tened, but resume their original form after the pressure is removed. 

 Their processes also are very elastic. 



An abundance of blood is distributed to the nerve-cells. The 

 vessels will be described in the second part of this chapter. 



Functions of the Nerve- Cells. 



Since the nerve-cells constitute the principal element in the gray 

 matter of the centres, and of the ganglia, and this presides over the 

 higher functions of the nervous system, while its other portions are 



