348 THE MECHANICAL FUNCTIONS. 



arm, although completely separate at an early period of 

 growth, soon unite to form but one bone. This union be- 

 gins 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 ap- 

 pears 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 afibrd 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 oc- 

 cur in these joints. The consolidation of parts is most con- 

 spicuous in the succeeding division of the limb, namely, 

 that constituting 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 cannon bone. In the early periods of ossification, how- 

 ever, they each consisted of two slender bones, lying close 

 and parallel to each other; but afterwards united by an ossi- 

 fic deposition, which fills up the interval between them, 

 and leaves behind no trace of suture.* In proportion as the 

 young animal acquires strength, the uni^n of these twcfbones 

 becomes still more intimate by the absodtttion of the parti- 

 tion which separated their cavities; so that ultimately they 

 constitute but one cylinder, with a single central cavity, 

 which is occupied by marrow. 



The cannon bone is much elongated, both in the fore and 

 hind extremity; so that the carpus and tarsus, which are the 

 commencements of the real feet, are raised considerably 



* The observations which establish this fact are detailed by G. St. Hilaire, 

 in a paper in the " Memoires du Museum," x. 173. 



