43G COMPARATIVE ANATOMY. 



are attached on their ventral surface. They may, however, also carry 

 terminal ribs, as in the posterior thoracic vertebras of the Cetacea. 

 As the thoracic pass into the lumbar vertebras the transverse process 

 is very commonly differentiated into three special processes. The 

 mamillary processes form knobs, which are sometimes of great 

 size, and which are directed forwards ; they may get to be placed 

 at the base of the anterior articular processes. The accessory 

 processes are directed backwards and upwards; and a third process 

 is lateral, though often directed downwards ; these form the 

 transverse processes (lateral processes) of the lumbar vertebra). 



The various divisions of the vertebral column are more sharply 

 differentiated from one another in Mammals than in Birds and 

 Reptiles. The cervical region is especially so ; it is distinguished by 

 the constant possession of seven vertebras, which are more definitely 

 marked off from the thoracic region, owing to the fact that their 

 rudimentary ribs rarely increase in size as they approach the thoracic 

 ribs. The increase in number of the cervical vertebras in Bradypus 

 to eight, or nine, is explained by the fact that the thoracic vertebras 

 pass into the cervical region, while the diminution to six in Cholcepus 

 and the American Manatee is similarly explained by the complete 

 development of the rib of the seventh cervical vertebra. 



The lumbar region is distinguished by the absence of movable 

 ribs. In the sacral region one vertebra only carries the ilium, as a 

 rule; but a second one has frequently the same relations. It is 

 more rarely that a third vertebra is connected with the ilium. The 

 fusion of these vertebras with one another, and with one or 

 more caudal vertebras, gives rise to the formation of a compact 

 region, which is known as the " Os sacrum ; " in this we must 

 distinguish the true sacral vertebras from the false ones which are 

 developed from the caudal vertebras. In the Edentata the number 

 of the sacral vertebras is increased by the union of the ischium with 

 the caudal vertebras. In this way the sacral region gets to extend 

 over eight or nine vertebras. 



In the Mammalia also the caudal region of the vertebral column 

 is the most variable of all regions; in most divisions it may be 

 either greatly developed, or may undergo considerable reduction. 

 Thus in the Simias the number of vertebras may be as high as thirty, 

 while in some again it may sink below the number, which has been 

 retained by man. 



In this point the last portion differs from the most anterior or 

 cervical region ; while the intermediate portion is, as regards its 

 numerical relations, less constant than the cervical, but at the same 

 time less variable than the caudal portion of the vertebral column. 

 The number of dorso-lumbar vertebras is very high in some of the 

 half-apes (23-24 in Lemur) ; in Cholcepus it is 27; in the Perisso- 

 dactyla, 24. It is highest in Hyrax (20). It is lower in all the other 

 divisions. 



Within the larger divisions the common ancestry of the different 

 genera is implied by the great constancy of the whole number of the 



