54 



APPARATUS OF MOTION. 



bones, called vertebrse, united to each other by ligaments. 

 Each vertebra has a solid centre with four branches, two of 

 A, which ascend and form an arch above, and 



two descend, forming an arch below the 

 body of the vertebra. The upper arches 

 form a cavity (a) which, along the region 

 of the trunk, encloses the spinal marrow, 

 and in the head receives the brain (61). 

 The lower arches (Z>) form a cavity, simi- 

 lar to the superior one, for containing the 

 organs of nutrition, and reproduction ; 

 they sometimes meet below, but generally 

 they remain separated, so that the inferior 

 cavity of the body is enclosed, in part, by 

 fleshy walls. Every part of the skeleton 

 may be reduced to this fundamental type, 

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

 the vertebrate animals ; so that between the pieces of 

 the head, the trunk, or the tail, we have only differences 

 in the degree of development of the body of the ver- 

 tebra, or of its branches, and not different plans of organ- 

 ization. 



162. The muscles which move this solid frame -work of 

 the vertebrata are disposed around the vertebrae, as is 



Fig. 28. 



Fig. 29. 



well exemplified among the fishes, where there is a band 

 of muscles for each vertebra. In proportion as limbs 



