360 THE MECHANICAL FUNCTIONS. 



While fleetness and elasticity are the results of the me- 

 chanical conformation of the horse, solidity and strength are 

 the objects chiefly aimed at in the construction of the pachy- 

 dermata. The limbs have a great weight to sustain, in con- 

 sequence of the huge size of the body; and hence the seve- 

 ral bones which compose the pillars for its support are ar- 

 ranged nearly in vertical lines. The joints of the elbow and 

 knee are placed low from the body; the ulna in the forelegs, 

 and the fibula in the hinder, are fully developed, and are 

 distinct from the radius and the tibia. The number of the 

 toes, instead of being reduced to one, as in the horse, or to 

 two, as in ruminants, is here increased to five: though, in 

 consequence of their being very short, and of the skin which 

 covers and surrounds them being very thick, they hardly 

 appear externally, and are distinctly recognised only in the 

 skeleton. 



It would carry me far beyond the limits of the present 

 work, were I to engage in a detailed examination of all the 

 varieties of forms and structures that occur in the mecha- 

 nism of the different tribes of mammalia, in reference to the 

 purposes they are intended to serve, and to the peculiar cir- 

 cumstances of the animal to which they belong. I must ne- 

 cessarily pass over a multitude of instances of express adap- 

 tation, which are suited only to particular cases, and are, 

 consequently, of minor importance as regards the general 

 plans of organization. In the sort of bird's-eye view which 

 I am taking of the endless modifications of structure that 

 have been executed in conformity with those plans, I am 

 able particularly to notice only such as are most remarkable. 



§ 8. Rodentia, 



As the tribes of mammalia we have hitherto examined, 

 employ the anterior extremities for the purposes of progres- 



man foetus, given rise to a monstrosity veiy much resembling' the trunk of the 

 tapir or of the" elephant. (See Geoffroy St. Hilaire.) 



