12 CELLS AND EPITHELIAL MEMBRANES. 



entirely or in part, by minute masses, or grains, of an 

 opaque and very dark colored substance (pigment), 

 such as are to be seen, for example, in the pigment 

 cells of the choroid, of the iris, and in many iierve- 

 cells. (PL II. fig. I, II., PI. XIII. fig. I. 6.) There 

 are cells, also, which in their normal condition con- 

 tain numerous minute spherical globules with a clearly 

 defined, dark outline, and possessing a very highly 

 refractive power ; these little pearl-like bodies are 

 nothing more than free fat, and are known as oil- 

 globules. Hepatic cells always contain a variable 

 quantity of them, and globules of colostrum are filled 

 with them. (PL I. fig. Ill, PL XVIII. fig. VI. 2.) 

 The same is true of the cells of sebaceous glands. 



The presence of oil globules in a cell which nor- 

 mally contains none, is evidence of its approaching 

 degeneration, and indicates arrest, or, at least, tem- 

 porary perversion of its physiological development. 

 This is to be seen in the pulmonary epithelial cells 

 while tubercular deposit is taking place ; it is also the 

 anatomical lesion of the cells of renal epithelium in 

 Bright's disease. Finally, crystals are sometimes 

 formed in the cells of adipose tissue. (PL II. fig. IV.) 



When cells, and especially young cells, are sub- 

 jected to the action of acetic acid under the micro- 

 scope, both the cell wall and its contents very soon 

 begin to grow pale, but their nuclei become more dis- 

 tinct; after a time the cell wall melts down and dis- 

 appears, but the nucleus retains its natural appear- 

 ance. If, in place of acetic acid, caustic potash 

 be applied in the same manner, even though largely 

 diluted, the cell begins at once to swell up, grows 



