130 HISTORY OF THE HUMAN BODY 



other animal is formed in accordance with a definite pattern 

 or is constructed upon any other principle save those of hered- 

 ity and environment. 



The anterior, or pectoral, girdle never becomes directly at- 

 tached to the vertebral column, and consequently the latter 

 receives no direct modification through the development of the 

 former, but the use of this region as a secondary center of 

 support causes a division of function between the vertebrae 

 that lie anterior and posterior to it^the first forming the neck. 

 By the establishment of this point and the sacrum, the two 

 centers of support, the vertebral column becomes divided into 

 regions, the differentiation of which depends upon the degree 

 of development of the limbs and the amount of difference in 

 the function to which these parts are subjected. Beginning 

 anteriorly the vertebrae anterior to the first center of support 

 are the cervical or neck vertebrae, the first one or two of which 

 are especially modified to bear the head and allow of its special 

 motions. The vertebrae between the shoulder-girdle and the 

 sacrum are spoken of in general as the trunk vertebra, and in 

 birds and mammals allow a further subdivision into thoracic 

 and lumbar, the former being provided with free ribs, and the 

 latter being without them. Then follow the sacral vertebra, 

 usually more than one in forms above the amphibia, followed 

 by the caudal vertebra or tail. The correlation between the 

 regional differentiation and the development of the limbs is 

 especially emphasized by such forms as the whales, which have 

 secondarily lost the hind limbs, and snakes, which have lost 

 both pairs, since in the former the deprived region, and in the 

 latter the entire vertebral column, have lost all trace of such 

 differentiation. 



The second cause of modification of the vertebral column is 

 correlated with the first and is directly due to the increase in 

 size of the limb muscles and their consequent need of broader 

 and stronger points of origin. The limb muscles, in the case 

 of animals with well-developed limbs, are usually broad, fan- 

 shaped sheets, like the trapezius and latissimus dorsi, attached 

 wholly or in part to certain processes of the vertebral column, 



