106 



THE SKELETON OP VEETEBRATA. 



solid parts at the surface, as the hairs and horns of mammals, 

 the coat of mail of the armadillo (fig. 75*), the feathers and 



claws of birds, the buck- 

 lers and scales of rep- 

 tiles and fishes, &c. But 

 they have, besides this, 

 along the interior of 

 the whole body, a solid 

 framework, not found 

 in the invertebrata, well 

 known as the SKELE- 



TON. 



Fig. 75*. External skeleton of the Dasypus 226. The skeleton is 

 sexcmctus. composed of a series of 



separate bones, called vertebrae, united to each other by liga- 

 ments. Each vertebra has a solid centre with several branches, 

 two of which ascend and form an arch above, and two descend, 

 forming an arch below the body of the vertebra. The upper arches 

 form a continuous cavity along the region of the trunk, which 

 encloses the spinal cord, and in the head receives the brain 

 ( 85 and 89). The lower arches form another cavity, similar 

 to the superior one, for containing the organs of nutrition and 

 reproduction ; the branches generally meet below, and when 

 disjoined, the deficiency is supplied by fleshy walls. Every 

 part of the skeleton may be reduced to this fundamental type, 

 the vertebra, as will be shown when treating specially of the 

 vertebrate animals; so that, between the pieces composing the 

 head, the trunk, and the tail, we have only differences in the de- 

 gree of development of the body of the vertebra, or of its 

 branches, and not in reality different plans of organization. 



227. The muscles which move this solid framework of the 

 vertebrata are disposed around the vertebrae, as is well exem- 

 plified among fishes, where there is a band of muscles for each 

 vertebra (fig. 76). In proportion as limbs are developed, this 

 intimate relation between the muscles and the vertebrae dimi- 

 nishes. The muscles are unequally distributed, and are con- 

 centrated about the limbs, where the greatest amount of 

 muscular force is required. For this reason the largest 

 masses of flesh, in the higher vertebrata, are found about the 

 shoulders and hips (fig. 77); while in fishes they are concen- 

 trated about the base of the tail, the part on which they princi- 

 pally depend for motion. 



