446 THFi MECHANICAL FUNCTIONS. 



form part of the trunk ; being entirely enveloped 

 and concealed by the large muscles connecting 

 them with the body. The heads of the two humeri, 

 in consequence of the absence of the clavicle, are 

 brought very near each other ; so as to occupy a 

 situation as nearly as possible underneath the 

 weight which the limb has to support. 



The radius and ulna, which are the two bones of 

 the fore arm, although completely separate at an 

 early period of growth, soon unite to form but one 

 bone. This union begins at their lower end, and 

 proceeds upwards to within a short distance from 

 the top, where a separation may still be observed 

 in the processes which project from that end, form- 

 ing for some way down a distinct suture. This 

 union of the two bones must, of course, preclude all 

 rotatory motion ; but it is calculated to give the 

 joint great security : and this appears to have been 

 the main object in the conformation of the whole 

 limb. The same process of consolidation takes 

 place in the hind leg, between the tibia and the 

 fibula, which are so completely united, as to afford 

 scarcely any trace of their having been originally 

 separate. 



The carpus and the tarsus are both of very 

 limited extent, and consist of a smaller number of 

 pieces than usually occur in these joints. The 

 consolidation of parts is most conspicuous in the 

 succeeding division of the limb, namely, that con- 

 stituting the metacarpus in the anterior, and the 

 metatarsus in the hind extremity. In either case, 

 we find it consisting, not of five bones, as in the 

 more highly organized carnivorous mammalia, but 

 of a single bone only, termed the ccnmou hone. In 



