108 ANIMAL MECHANISM. 



resistance. This variability depends on many causes. Thus, 

 different kinds of animals, when they move, have not to 

 struggle with the same effort against their weight. The fish, 

 which is of nearly the same specific gravity as water, finds 

 itself su-pended in it without having to exert any force; 

 and if it wishes to move in any direction, it lias only to 

 overcome the resistance of the fluid which it is necessary 

 to displace. The bird, on the contrary, if it desires to sus- 

 tain itself in the air, must make an effort capable of 

 neutralizing the action of its weight. If it moves forward 

 at the same time, it must perform, in addition, the work 

 which is consumed in overcoming the resistance of the air. 



Partition of muscular force between the points of resistance and 

 the muss of the lodij. When, in physiology, we seek to es- 

 timate the work of a muscle, we fix it firmly by one of its 

 attachments, and we ascertain the extent passed through 

 by its movable extremity. If we know the weight which 

 this muscle can raise as it contracts, and the extent through 

 which that weight is raised, we have elements by which we 

 can estimate the work effected. But these are almost ideal 

 conditions, which are scarcely ever found in terrestrial loco- 

 motion; nor can we observe them in animals which move in 

 the water, and more especially in those which fly through the 

 air. Let us only compare the effort necessary to walk on a 

 movable soil, on sandy dunes, for instance, with that required 

 in walking on firm soil. We shall see that the mobility of 

 iho resisting surface presented by the sand destroys a part of 

 the dlort necessary for the contraction of our muscles; in 

 other words, that a greater eff.rt is necessary to produce the 

 same useful work, when the point of resistance is not stable. 



This amount of work is easy to be understood, and even to 

 be measured. 



When a man, while walking, places one of his feet on the 

 ground, the corresponding leg, slightly bent, draws itself up, 

 and pressing on the ground In-low, gives at the same time an 

 upward impulse to the body. If the ground entirely resist 

 this pressure, all the movement produced will be in tho 

 dim-lion ol' the trunk of the body, which will be raised 

 to a certain height, three centimetres for example. But if 



