128 HISTORY OF THE HUMAN BODY 



a sinuous motion of the body. Even here, however, the influ- 

 ence of the limbs would be felt, since they would cling to the 

 surface and thus furnish definite fixed points where the sinu- 

 ous motion of the vertebrae would be lessened. By a gradual 

 increase in the size and strength of the limbs, the animal would 

 attain the power of crawling, that is, of dragging the body 

 over the ground through the action of the limb muscles as well 

 as those of the back, and finally the limbs would become strong 

 enough to bear the entire weight and the body would be lifted 

 wholly above the surface of the ground, thus changing the 

 crawling motion into a true walk. This gradual development 

 of the free limbs is accompanied by important correlated 

 changes in the vertebral column, due in the main to two causes. 

 The first of these is the increased size and functional impor- 

 tance of the limb girdles, or those parts of the limb skeleton 

 enclosed within the body, to which the free part is movably 

 attached, usually by a ball-and-socket joint; and the second is" 

 the increase in size of the proximal limb muscles. As the 

 limbs become larger and stronger, their girdles, i. e. } the proxi- 

 mal portion of their skeleton, feel the need of a stronger sup- 

 port and a more intimate association with the axial skeleton, 

 a need especially felt by the hip-girdle, since here the greater 

 weight is sustained. This girdle, seeking the necessary sup- 

 port, grows dorsally around the body, until it meets the ends 

 of a pair of ribs with which it articulates. In the lower 

 forms the ribs are very short and are borne upon the end of 

 short transverse processes, and the girdle with its dorsally 

 developing process, known as the ilium, the rib, the transverse 

 process, and the vertebra, form a complete chain around the 

 body. 



At first this association involves but a single vertebra, the 

 location of which is apt to vary. Thus in Necturus] the most 

 primitive salamander now in existence, the hip-girdle is usually 

 attached to the iQth vertebra, counting from the head, but the 

 2Oth is occasionally employed instead, and two cases have been 

 reported in which the attachment was to the i8th. Cases are 

 also known in which the attachment is oblique, either to the 



