472 STRUCTURE AND CONNEXIONS OF VERTEBRA. 



628. It has been already noticed (71) that an ordinary 

 character of the vertebra consists in their being perforated by 

 an aperture (fig. 225), which, when several vertebrae are united. 



together, forms a continuous tube or canal for 

 the lodgment of the spinal cord. This cha- 

 racter is usually lost, however, in the coccy- 

 geal vertebrae ; which are so much contracted 

 and simplified as to contain no aperture. 

 The purpose of the division of the spinal 

 column into so large a number of separate 

 Fig. 225. SINGLE bones, is obviously to allow of considerable 

 freedom of motion by a slight shifting amongst 

 the individual parts ; whilst any such sudden bend as would 

 be injurious to the spinal cord, is avoided. Each vertebra con- 

 sists of a solid " body " a, which is situated in front of the 

 spinal canal in Man, but below it in animals whose back has 

 a horizontal position, and which serves to give solidity to the 

 structure, and of "processes" or projections, b and c, that 

 serve to form the spinal canal, and to unite the vertebra to 

 each other. In Man and other warm-blooded animals, the 

 two surfaces of the "body" are nearly flat and are parallel to 

 each other ; and they are united to the corresponding surfaces 

 of the neighbouring vertebrae by a disk of fibro-cartilage (47), 

 which extends through the whole space that intervenes be- 

 tween them, and which, being firmly adherent to both, prevents 

 them from being far separated from each other. 



629. But in Reptiles and Fishes, a different plan is adopted. 

 In the animals of the former class, particularly in Serpents, 

 we find one surface of each vertebra convex or projecting, and 

 the other concave or hollowed-out ; and the convex surface of 

 each vertebra fits into the concave surface of the next, in such 

 a manner that the whole spinal column becomes a series of 

 ball-and-socket joints, and is thus endowed with that flexibi- 

 lity which is essential to the peculiar movements of these 

 animals. In Fishes both surfaces are concave, and between 

 each vertebra there is interposed a bag containing fluid, and 

 having two convex surfaces, over which those of the vertebrae 

 can freely play. Extreme facility of movement is thus given 

 to the spinal column ; but its strength is proportionally dimi- 

 nished. It is to be remembered, however, that strength is 

 not required in the bony framework of animals, whose bodies, 



