THE SKELETON 13 



number — since they are merged into one bony- 

 mass in which are also incorporated the last 

 thoracic and the first coccygeal vertebrae.^ 

 The composite bone has, at first, a ridge 

 representing the fused spinous processes, but 

 this disappears about the middle of the bone. 

 On the ventral surface, ridges indicate the 

 position of the transverse processes. 



The coccygeal region contains five or six 

 bones, of which the last — produced by the 

 union of several vertebrae — is the largest, is 

 known as the pygostyle, and forms a founda- 

 tion for the feathers of the tail. 



Of the seven pairs of ribs the first and 

 second, and sometimes the seventh, do not 

 reach the sternum. Each of the other ribs 

 consists of two segments — a vertebral and 

 a sternal. The proximal end of each rib con- 

 sists of a head, separated from the rest of 

 the bone by a neck, and a tubercle. The head 

 articulates with the body and the tubercle 

 with the transverse process of a vertebra. 

 Most of the ribs — the first and last are excep- 



1 It seems probable that the sacrum is originally composed of 

 two vertebrae only, with which all the lumbar and a considerable 

 proportion of the coccygeal vertebrae subsequently coalesce. The 

 development of this part of the vertebral column of the chick has 

 not received sufficient attention to allow of a definite statement. 



