MUSCLE AND NERYE. 29 



given off; or help in the evacution, or fertilization, or develop- 

 ment, of these germs. 



On the other hand, the correlative functions, so long as 

 they are exerted by a simple undifferentiated morphological 

 unit or cell, are of the simplest character, consisting of those 

 modifications of position which can be effected by mere 

 changes in the form or arrangement of the parts of the pro- 

 toplasm, or of those prolongations of the protoplasm which 

 are called pseudopodia or cilia. But, in the higher animals 

 and plants, the movements of the organism and of its parts 

 are brought about by the change of the form of certain tis- 

 sues, the property of which is to shorten in one direction 

 when exposed to certain stimuli. Such tissues are termed 

 contractile ; and, in their most fully developed condition, 

 muscular. The stimulus by which this contraction is natu- 

 rally brought about is a molecular change, either in the sub- 

 stance of the contractile tissue itself, or in some other part 

 of the body ; in which latter case, the motion which is set up 

 in that part of the body must be propagated to the contractile 

 tissue through the intermediate substance of the body. In 

 plants, there seems to be no question that parts which retain 

 a hardly modified cellular structure may serve as channels for 

 the transmission of this molecular motion ; wdiether the same 

 is true of animals is not certain. But, in all the more com- 

 plex animals, a peculiar fibrous tissue — nerve — serves as the 

 agent by w4iich contractile tissue is affected by changes oc- 

 curring elsewhere, and by which contractions thus initiated 

 are coordinated and brought into harmonious combination. 

 While the sustentative functions in the higher forms of life 

 are still, as in the lower, fundamentally dependent upon the 

 powers inherent in all the physiological units which make up 

 the body, the correlative functions are, in the former, deputed 

 to two sets of specially modified units, which constitute the 

 muscular and the nervous tissues. 



When the different forms of life are compared together as 

 physiological machines, tliey are found to differ as machines 

 of human construction do. In the lower forms, the mechan- 

 ism, though perfectly well adapted to do the work for which 

 it is required, is rough, simple, and weak ; while, in the 

 higher, it is finished, complicated, and powerful. Considered 

 as machines, there is the same sort of difference between a 

 polyp and a horse as there is between a distaff and a spin- 

 ning-jenny. In the progress from the lower to the higher 

 organism, there is a gradual differentiation of organs and of 



