THE PIGEON 199 



The axis is an elongated vertebra, projecting from the anterior 

 end of which is the cylindrical odontoid process ; this process is 

 equivalent to the centrum of the altas. 



Exercise 40. Draw the lateral aspect of the sixth cervical vertebra 

 on a scale of 2 ; draw the anterior aspect. 



Exercise 41. Draw the anterior aspect of the atlas on a scale of 2. 



Exercise 42. Draw the lateral aspect of the atlas and axis on a 

 scale of 2. 



The thoracic vertebrae are five in number, being those which are 

 joined with the sternum by ribs. They are all fused together 

 and with the lumbar vertebrae. The posterior thoracic vertebra is 

 fused also with the pelvis. 



The three lumbar vertebrae, the four sacral vertebrae, and the 

 six anterior caudal vertebrae are all fused with one another and 

 with the pelvis. These vertebrae, together with the fifth thoracic 

 vertebra, form what is called the synsacrum. 



Note carefully the character of these fused vertebrae, especially 

 of the transverse processes. Note the intervertebral foramina 

 through which the spinal nerves pass. 



The free caudal vertebrae are six in number. The spinal column 

 ends posteriorly with the pygostyle, an irregular bone formed by 

 the fusion of several vertebrae. 



The Ribs. The first two ribs articulate with the last two cer- 

 vical vertebrae and do not reach the sternum. Each has two artic- 

 ular surfaces: the head, or capitulum, which articulates with the 

 centrum ; and the tuberculum, which articulates with the trans- 

 verse process. The five posterior ribs articulate with the thoracic 

 vertebrae and also with the sternum. Each thoracic rib has two 

 portions,— a dorsal vertebral portion and a ventral sternal portion, 

 which articulate with each other. Projecting backward from the 

 vertebral portions of all the ribs except the first and the last is 

 the prominent uncinate process. 



The sternum, or breastbone, is a large, flat bone which extends 

 far back of the ribs and covers the ventral side of the breast and 

 a large part of the abdomen. On its ventral surface is the keel, 

 which projects ventrally in the median plane. Inasmuch as the 



