MI'SCLE AND >., 



given off; or help in the evacution, or fertilizati>, . clop- 



incut, of these germs. 



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

 they an- exerted by a simple lindifVcrentiatcd mm pin. logical 

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

 modifications of position which can i by mere 



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

 toplasm, or of those prolongations of the protoplasm \\ 

 arc 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 certar 

 sues, the property of which is to shorten in one 

 \\hcn exposed to certain stimuli. Such tissues an 

 contractile; and, in their most fully developed condr 

 muscular. The stimulus by which this contraction is i, 

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

 stance of the contractile tissue itself, or in sn me 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 cont ra< tile 

 tissue through the intermediate substance of the bodv. In 

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

 a hardly modified cellular structure may serve as channels for 

 the transmission of this molecular motion ; whether the same 

 is true of animals is not ccitain. But, in all the more < 

 plex animals, a peculiar fibrous tissue nerve serves as the 

 agent by which contractile tissue is affected by changes oc- 

 curring elsewhere, and by which contractions thus initiated 

 are coordinated and brought into harmonious coml-in. 

 While the sustentative functions in the higher forms of life 

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

 powers inherent in all the physiological units which make up 

 the body, the correlative functions are, in the t'orn:cr, 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, they are found to differ as marl 

 of human construction do. In the lower forms, the m< i -han- 

 ism, though perfectly well adapted to do the work f< r \\hich 

 it is required, is rough, simple, and weak; \vhile. in the 

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

 as machines, there is the same sort of difference 1 . 

 polyp and a horse as there is between a <li>ra|f and a spin- 

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

 organism, there is a gradual differentiation of organs and of 



