DEVELOPMENT OF THE SKELETON. 



395 



Fio. 41. 



to the twelfth year. 1 The processes of development just de- 

 scribed are represented in Fig. 40. 



Vertebral Column, Etc. In Fig. 38 and 39 (<?), are seen the 

 two masses of cells, situated by the sides of the neural canal, 

 which are destined to be developed into the vertebrae. These 

 cells extend around and encroach upon the chorda dorsalis, 

 and form the bodies of the vertebrae. They also extend 

 over the neural canal, closing above, and these processes are 

 called the medullary, or dorsal plates. Sometimes the dorsal 

 plates fail to close at a certain 

 point in the spinal column, and 

 this constitutes the malformation 

 known as spina bifida. From the 

 sides of the bodies of the ver- 

 tebrae, the various processes of 

 these bones are formed. As the 

 spinal column is developed, its 

 lower portion presents a projec- 

 tion beyond the pelvis, which 

 constitutes a temporary caudal 

 appendage, curved toward the 

 abdomen ; but this no longer 

 projects after the bones of the 

 pelvis are fully developed. At 

 the same time, the entire verte- 

 bral column is curved toward the abdomen, and it is twisted 

 upon its axis from left to right, so that the anterior face of 

 the pelvis presents a right angle to the upper part of the 

 body ; but, as the inferior extremities and the pelvis become 

 developed, the spine becomes straight. The vertebrae make 

 their appearance first in the middle of the dorsal region, from 

 which point, they rapidly extend upward and downward, until 

 the spinal column is complete. 



At the base of the skull, on either side of the superior 



1 ROBIN, Memoire sur revolution de la notocorde, Paris, 1868, p. 10. 



Human embryo, about one month old, 

 showing the large size of the head 



and upper parts of the body, the 

 twisted form of the spinal column, 

 the rudimentary condition of the up- 



per and lower extremities, and the 

 rudimentary tail at the end of the 

 spinal column. (DALTON, Human 

 Physiology, Philadelphia, 1871, p. 

 663.) 



