

DEVELOPMENT OF THE SKELETON 



S3 



body of the vertebra, and forms the antero-lateral boundary of the foramen. The 

 distal portions of the primitive costal arches remain undeveloped; occasionally 

 the arch of the seventh cervical vertebra undergoes greater development, and by 

 the formation of costovertebral joints is separated off as a rib. In the lumbar 

 region the distal portions of the primitive costal arches fail ; the proximal portions 

 fuse with the transverse processes to form the transverse processes of descriptive 

 anatomy. Occasionally a movable rib is developed in connection with the first 

 lumbar vertebra. In the sacral region costal processes are developed only in 

 connection with the upper three, or it may be four, vertebrae; the processes of 

 adjacent segments fuse with one another to form the lateral parts of the sacrum. 

 The coccygeal vertebra are devoid of costal processes. 



CERVICAL 



LUMBAR 



THORACIC 



SACRAL 



FIG. 67. Diagrams showing the portions of the adult vertebrae derived respectively from the bodies, vertebral 

 arches, and costal processes of the embryonic vertebra. The bodies are represented in yellow, the vertebral arches 

 in red, and the costal processes in blue. 



The Sternum. The ventral ends of the ribs become united to one another by a 

 longitudinal bar termed the sternal plate, and opposite the first seven pairs of ribs 

 these sternal plates fuse in the middle line to form the manubrium and body of the 

 sternum. The xiphoid process is formed by a backward extension of the sternal 

 plates. 



The Skull. Up to a certain stage the development of the skull corresponds with 

 that of the vertebral column; but it is modified later in association with the expan- 

 sion of the brain-vesicles, the formation of the organs of smell, sight, and hearing, 

 and the development of the mouth and pharynx. 



