STRUCTURE OF THE VERTEBRAL COLUMN. 



severe injury, from the sudden bends which its bony casing would 

 have been forced to make. 



66. The flexibility of the spinal column varies 

 much in degree, however, in different species of 

 Yertebrata. In Fishes it is very great ; the body 

 being chiefly propelled by the strokes of its 

 hinder part and tail from side to side, and the 

 side fins having but little else to do than to direct 

 its movement. In Serpents, we find an entire 

 absence of members or extremities ; and the whole 

 of the movement of the trunk is effected by itself 

 alone ; consequently the spine requires great flex- 

 FIG. 43" VKRTE- ibility. In these two groups we find an especial pro- 

 BRAL COLUMN, vision for this object ; for in Serpents, the bodies of 

 the vertebras are not flat, but are convex on one side and concave 

 on the other, so as to form a series of ball-and-socket joints, 

 admirably adapted to give the greatest flexibility ; and in fishes, 

 both surfaces of each vertebra are concave, and work upon a 

 nearly globular bag of fluid, which is interposed between them. 

 Such an arrangement as this last weakens the spine to a degree, 

 which would prevent its being ever employed in a land animal ; 

 in which the weight of the body is thrown upon only four points, 

 and in which there must be a certain resistance in the connecting 

 framework : but in Fishes, in which every part of the body is 

 equally buoyed up by the surrounding element, there is no neces- 

 sity for such power of resistance ; and flexibility is the great 

 object to be attained. In some of the lowest Fishes, such as the 

 Lamprey (Fig. 44), there is no proper vertebral column ; the 



FIG. 44. LAMPREY. 



spinal cord being inclosed in a flexible cartilaginous tube, the 

 division of which into segments is only marked by some cross 

 wrinkles : and in one species, even this is wanting, the spinal 

 cord being only surrounded by a membranous envelope. Such 



