FUNCTIONS AND ORGANS. 27 



this aspect, the phenomena which they present have no par- 

 allel in the mineral world. 



The actions of living matter are termed its functions ; 

 and these functions, varied as they are, may be reduced to 

 three categories. They are either — (1), functions which affect 

 the material composition of the body, and determine its mass, 

 which is the balance of the processes of waste on the one 

 hand and those of assimilation on the other ; or (2), they are 

 functions which subserve the process of reproduction, which 

 is essentially the detachment of a part endowed with the pow- 

 er of developing into an independent whole ; or (3), they are 

 functions in virtue of which one part of the body is able to 

 exert a direct influence on another, and the body, by its parts 

 or as a whole, becomes a source of molar motion. The first 

 may be termed sustentative, the second generative, and the 

 third correlative functions. 



Of these three classes of functions the first two only can 

 be said to be invariably present in living beings, all of which 

 are nourished, grow, and multiply. But there are some forms 

 of life, such as many Fungi, which are not known to possess 

 any powers of changing their form ; in which the protoplasm 

 exhibits no movements, and reacts upon no stimulus ; and in 

 which any influence which the different parts of the body ex- 

 ert upon one another must be transmitted indirectly from 

 molecule to molecule of the common mass. In most of the 

 lowest plants, however, and in all animals yet known, the 

 body either constant^ or temporarily changes its form, either 

 with or without the application of a special stimulus, and 

 thereby modifies the relations of its parts to one another, and 

 of the whole to surrounding bodies ; while, in all the higher 

 animals, the different parts of the body are able to affect, and be 

 affected by one another, by means of a special tissue, termed 

 nerve. Molar motion is effected on a large scale by means of 

 another special tissue, muscle y and the organism is brought 

 into relation with surrounding bodies by means of a third 

 kind of special tissue — that of the sensory organs — by means 

 of which the forces exerted by surrounding bodies are trans- 

 muted into affections of nerve. 



In the lowest forms of life, the functions which have been 

 enumerated are seen in their simplest forms, and they are ex- 

 erted indifferently, or nearly so, by all parts of the proto- 

 plasmic body ; and the like is true of the functions of the 

 body of even the highest organisms, so long as they are in 

 the condition of the nucleated cell, which constitutes the 



