36 ^« Introduction to Zoology. 



In the geueral principles of all that class of functious which we have 

 hitherto considered, those merely connected with nutrition, and including 

 the aeration of the nutritive fluid, its diffusion through the system, and 

 its conversion into appropriate substances j animals, as we have already 

 observed, only participate in functions which are involved in the necessary 

 conditions of all life, and are therefore also common to vegetables. But 

 since, as we have already seen, animals do not derive that nutriment at 

 once from the surrounding elements in a state of nearly complete prepara- 

 tion, but from diverse kinds of food which require a preliminary apparatus 

 of concoction j from this, and from similar causes, a still more important 

 and marked difference, which in fact constitutes the essential distinction 

 between animal and vegetable life, namely, the necessity for the superad- 

 dition of voluntary motion, perception, and volition, to originate and direct 

 those motions, arises. For since the animals have to provide thems^ves 

 with these articles of necessary food, the very possibility of their continued 

 existence requires, that they should be gifted with powers which may 

 enable them to detect, to apprehend, and to convey into their alimentary 

 canal, the substances which they are destined to convert into nourishment. 

 We may therefore next proceed to consider these powers ; and first, we 

 may review the mechanical instruments of those motions ; the muscular, 

 and its support, the osseous system ; and then proceed to the nervous 

 power, the seat of that perception and volition which elevates the animal 

 frame, from a mere material to a vital structure, and causes it to become 

 " this sensible warm motion." Here, indeed, the power is in itself the 

 most profound of all mysteries, but we can nevertheless investigate the 

 material nervous organisation with which it is obviously connected. 



The prime moving power of the animal system, is that which is deno- 

 minated the contractility of its parts. The illustrations which we shall 

 proceed to give will, we believe, be found in some degree original, and as 

 such, we particularly invite to them the attention of our readers. Its 

 organs are fasciculi of fibres, called muscles, in all but the very lowest orders 

 of animal existence ; and we must postpone our remarks on these till we 

 have first considered the function where the mechanism by which it acts 

 is more palpably di.=played. Such a fasciculus may well be represented by 

 a skein of thread ; and muscular contractility would be illustrated by a 

 close analogy, if we were to connect such a skein with the conductor of an 

 electrical machine : the fibres would in that case be made strongly to repel 

 each other, and a consequent change in its dimensions must ensue ; if it 

 were suspended vertically with a weight attached by a string to its lower 

 extremity, the repulsion would cause a divergence in the breadth of the 

 threads, which would shorten the length of the skein, and raise the weight 

 by the string. Here the skein will represent the muscle, the string the 

 tendon, and the weight the limb to be moved. Again, the skein might be 



