GENERAL STRUCTURE OF ANIMALS. 23 



all performing similar, i. e., all the functions, but imper- 

 fectly. Then the process of differentiation commences 

 and proceeds. Some cells take on a special form 

 adapted to the performance of a special function, say 

 contraction, and aggregate to form a tissue, muscle. 

 Other cells take other forms and aggregate to form 

 other tissues adapted to perform other characteristic 

 functions, until finally in the mature condition of the 

 highest animals each kind of cell performs but one func- 

 tion, but performs it very perfectly. Thus a muscular 

 fiber can do nothing else but contract. A nerve cell 

 gives no other sign of life but feeling, etc. This whole 

 process of modification of form and limitation and per- 

 fecting of function, or division of labor, is called the 

 law of differentiation. It is the most fundamental and 

 universal law of evolution. 



Observe here — and the same is true of all differentia- 

 tions — two ideas which must be kept distinct in the mind, 

 viz., (1) identity of plan or community of origin — in this 

 case cellular structure — and (2) adaptive modification for 

 various functions. 



So much for tissues. But physiology is concerned 

 with functions, and functions are usually and properly 

 treated in connection with organs, such as muscle, brain, 

 gland, etc. The body consists primarily of organs. 

 Thus cells aggregate to form tissues, tissues aggregate 

 to form organs, and organs aggregated form the animal 

 body. 



SECTION IV. 



ORGANS AND FUNCTIONS OF THE ANIMAL BODY. 



Classification of Functions. — The functions of 

 the animal body are of two general kinds, viz., functions 

 of animal life and functions of vegetative or organic life, 



