MAMMALIA 259 



bones for the support of the hinder pair of limbs. The centra, 

 the transverse processes, with which rib rudiments are fused, 

 and the articular processes are fused, but the spines are 

 usually left free. The wings of the sacrum provide a pair of 

 surfaces for articulation with the innominates. 



The caudal or coccygeal vertebrae, as we follow the series 

 from before backwards, gradually lose their processes and 

 their arches, and finally are reduced to rod-like centra. 



The vertebral centra increase in diameter by the action of 

 the periosteum, and in length by the activities of the carti- 

 laginous junction between the body and the flat epiphyses. 

 The increase in size of the arches is provided for similarly by 

 cartilages at their bases. Between the vertebrae are discs of 

 cartilage which allow only of a limited degree of movement, 

 which is assisted by the gliding movements permitted by the 

 articular processes. The vertebrae give attachment to muscles, 

 and are firmly connected by ligaments which are elastic. The 

 tips of the spines are connected by the supraspinous ligaments, 

 and these, when they reach the long spines of the thoracic region, 

 extend forward to be attached to the occipital region of the 

 skull. It thus forms the ligamentum nuchae which serves to 

 support the head, holding it in a horizontal position in front 

 of the body, and a change of this position involves muscular 

 action. Interspinous ligaments unite the successive spines, 

 and a similar flat sheet unites the ligamentum nuchae to the 

 cervical vertebrae. The ligamenta subflava unite the suc- 

 cessive arches of the vertebrae, and other ligaments are related 

 to the centra and the articular processes. The articulations 

 of the separate elements of the column thus provide for move- 

 ment. The movements are very limited in the thoracic region, 

 are more pronounced in the lumbar, still more so in the neck, 

 and especially in the tail. With the exception of the rotatory 

 movements, specially provided between the first two vertebrae, 

 the bending produced by the vertebral column is performed 

 by the co-operation of a succession of vertebrae. 



The ribs are preformed in cartilage, and the cartilage is 

 only partially replaced by bone. The ribs are bony above, and 

 are continued downwards in cartilage. In the rabbit the first 

 seven are articulated by their cartilaginous lower extremities 



