78 



APPARATUS OF MOTION. 



a 



Each vertebra has a solid centre with four 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 (a) along 

 the region of the trunk, which encloses 

 the spinal marrow, and in the head re- 

 ceives the brain, (61.) The lower arches 

 (Z>) form another cavity, similar to the 

 superior one, which contains the organs of 

 nutrition and reproduction ; their branch- 

 es generally meet below, and when dis- 

 joined, 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, 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 in reality different plans 

 of organization. 



162. The muscles which move this solid framework 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 



