8 COMPARATIVE PHYSIOLOGY. 



not only m different groups of cells, but often in the same cell, 

 so that we can distinguish an outer portion (ectoplasm) and an 

 inner more fluid and more granular region {endoplasm). 



The nucleus is a body with very clearly defined outline (in 

 some cases limited by a membrane), through which an irregular 

 network of fibers extends that stains more deeply than any 

 other part of the whole cell. 



Owing to the fact that it is so readily changed by the action 

 of reagents, it is impossible to ascertain the exact chemical com,- 

 position of living protoplasm ; in consequence, we can only 

 infer its chemical structure, etc., from the examination of the 

 dead substance. 



In general, it may be said that protoplasm belongs to the 

 class of bodies known as proteids — that is, it consists chemically 

 of carbon, hydrogen, a little sulphur, oxygen, and nitrogen, ar- 

 ranged into a very complex and unstable molecule. This very 

 instability seems to explain at once its adaptability for the man- 

 ifestation of its nature as living matter, and at the same time the 

 readiness with which it is modified by many circumstances, so 

 that it is possible to understand that life demands an incessant 

 adaptation of internal to external conditions. 



It seems highly probable that protoplasm is not a single pro- 

 teid substance, but a mixture of such ; or let us rather say, fur- 

 nishes these when chemically examined and therefore dead. 



Very frequently, indeed generally, protoplasm contains other 

 substances, as salts, fat, starch, chlorophyl, etc. 



From the fact that the nucleus stains differently from the 

 cell contents, we may infer a difference between them, physi- 

 cal and especially chemical. It (nucleus) furnishes on analysis 

 nuclein, which contains the same elements as protoplasm (with 

 the exception of sulphur) together with phosphorus. Nuclei 

 have great resisting power to ordinary solvents and even the 

 digestive juices. 



Inasmuch as all vital phenomena are associated with proto- 

 plasm, it has been termed the " physical basis of life " (Hux- 

 ley). _ 



Tissues. — A collection of cells performing a similar physio- 

 logical action constitutes a tissue. 



Generally the cells are held together either by others with 

 that sole function, or by cement material secreted by them- 

 selves. Au organ may consist of one or several tissues. Thus 

 the stomach consists of muscular, serous, connective, and gland- 



