COMPAEATIVE ANATOMY. 201 



in the contrary direction to the antagonism immediately above it, thus 

 intensifying the stability. 



Homologies of the Limbs of different Animals. -Com- 

 paring the anterior limbs of different orders of mammals with those of 

 birds, reptiles and fishes (See Fig. 345) the same fundamental idea will 

 be recognized in all, yet so modified as to adapt the several species to the 

 most diversified modes of life. Thus from man to the fish are found the 

 humerus (H), the radius and ulna of the forearm (BU), the carpal 

 bones (C), the metacarpal (MC), and the phalanges (PH). The greatest 

 modification is observable in the wrist-bones (indicated by the dotted 

 line) and those beyond. The normal number of carpals, metacarpals, 

 and phalanges, is sometimes reduced and sometimes multiplied, that 

 the limb may be used specifically, according to the wants of the 

 animal, for swimming, crawling, flying, paddling, running, digging, 

 climbing, etc. The perfection of the human hand, which combines 

 nearly all these uses, depends chiefly on the free revolving motion of 

 the humerus and the rotation of the radius about the ulna. This mode 

 of action necessitates a clavicle. The more perfect this bone, the 

 greater the power of rotating the forearm ; and, when the clavicle is 

 wanting, the radius and ulna are usually consolidated and the fore limbs 

 become only columns of support or instruments of progression. This 

 latter condition existing in the horse and the ruminants, the meta- 

 carpals are greatly elongated, elevating the beel to the height of the knee 

 in other quadrupeds, and permitting the toe only to touch the ground. 



The posterior extremities follow the analogies of the anterior in 

 most mammals, the true knee with its patella being more or less con- 

 cealed in the flanks, with a corresponding elongation of the heel, the 

 toe only touching the ground. 



Laws of Comparative Anatomy.— The foregoing remarks 

 upon homologies and analogies prepare for a deduction of some of 

 those great laws of coexistence, whiph lie at the foundation of com- 

 parative anatomy. 



1. Since the extremities by means of which the animal procures its 

 food must always bear a certain relation to the teeth which masticate 

 and fit it for the stomach, the former decide the animal's external 

 structure, and the latter its internal organization. 



2. The position and size of the processes of different bones deter- 

 mine the direction and force of the muscles by which they are worked. 



3. The character of the muscles determines the structure of the 

 nerves which call them into action, and of the brain in which the 

 nerves originate. 



4. The nervous system determines the degree of intelligence. 



The structure of an animal, its natural habit and mode of life, can 

 be deduced from the bones of the extremities, and even from the 



