22 



ANIMAL LOCOMOTION. 



While the bones of animals form levers and fulcra for portions 

 of the muscular system, it must never be forgotten that the 

 earth, water, or air form fulcra for the travelling surfaces of 

 animals as a whole. Two sets of fulcra are therefore always 

 to be considered, viz. those represented by the bones, and 

 those represented by the earth, water, or air respectively. 

 The former when acted upon by the muscles produce motion 

 in different parts of the animal (not necessarily progressive 

 motion) ; the latter when similarly influenced produce loco- 

 motion. Locomotion is greatly favoured by the tendency 

 which the body once set in motion has to advance in a straight 

 line. The form, strength, density, and elasticity of the skele- 

 ton varies in relation to the bulk and locomotive power of 

 the animal, and to the media in which it is destined to move. 



" The number of moveable articulations in a skeleton de- 

 termines the degree of its mobility within itself; and the 

 kind and number of the articulations of the locomotive organs 

 determine the number and disposition of the muscles acting 

 upon them. 



The bones of vertebrated animals, especially those which 

 are entirely terrestrial, are much more elastic, hard, and 

 calculated by their chemical elements to bear the shocks and 

 strains incident to terrestrial progression, than those of the 

 aquatic vertebrata ; the bones of the latter being more fibrous 

 and spongy in their texture, the skeleton is more soft and 

 yielding. 



The bones of the higher orders of animals are constructed 

 according to the most approved mechanical principles. Thus 

 they are convex externally, concave within, and strengthened 

 by ridges running across their discs, as in the scapular and 

 iliac bones ; an arrangement which affords large surfaces for 

 the attachment of the powerful muscles of locomotion. The 

 bones of birds in many cases are not filled with marrow but 

 with air, — a circumstance which insures that they shall be 

 very strong and very light. 



In the thigh bones of most animals an angle is formed by 

 the head and neck of the bone with the axis of the body, 

 which prevents the weight of the superstructure coming 

 vertically upon the shaft, converts the bone into an elastic 



