68 LAND MAMMALS IN THE WESTERN HEMISPHERE 



bones called vertebrae. These are so articulated together as to 

 permit the necessary amount of flexibility and yet retain the 

 indispensable degree of strength. The function of the back- 

 bone is a twofold one : (1) to afford a firm support to the body 

 and give points of attachment to the limbs, and (2) to carry the 

 spinal cord, or great central axis of the nervous system, in such 

 a manner that it shall be protected against injury, a matter of 

 absolutely vital necessity. 



While the vertebrae differ greatly in form and appearance in 

 the various regions of the neck, body and tail, in adaptation 

 to the various degrees of mobility and strength which are 

 required of them, yet they are all constituted upon the same 

 easily recognizable plan. The principal mass of bone in each 

 vertebra is the body, or centrum, which is typically a cylinder, 

 or modification of that form, and the two ends of the cylinder 

 are the faces, by which the successive vertebrae are in contact 

 with one another. In the living animal, however, the successive 

 centra are not in actual contact, but are separated by disks of 

 cartilage (gristle) which greatly add to the elasticity of the 

 column. From the upper surface of the centrum arises an 

 arch of bone, the neural arch, enclosing with the centrum the 

 neural canal, through which runs the spinal cord. As already 

 mentioned, the protection of the spinal cord is essential to the 

 life of the animal, yet this protection must be combined with 

 a certain flexibility, both lateral and vertical. Mere contact 

 of the centra, even though these be held in place by ligaments, 

 would not give the column strength to endure, without dis- 

 location, the great muscular stresses which are put upon it. 

 Additional means of articulation between the successive 

 vertebrae are therefore provided, and these vary in size, form 

 and position in different regions of the backbone, in nice adjust- 

 ment to the amount of motion and degree of strength needed 

 at any particular part of the column. Of these additional 

 means of articulation, which are called the zygapophyses, each 

 vertebra has two pairs, an anterior and a posterior pair, placed 



