264 VERTEBRATE ZOOLOGY 



this rib rudiment and the transverse process is a round hole, the 

 foramen transversarium. Note the shape of the transverse proc- 

 esses in the different cervical vertebrae. The dorsal portion in each 

 case represents the transverse process proper ; the ventral portion 

 represents the rib rudiment, and in the sixth vertebra is double. 

 In the seventh vertebra these ribs, and consequently the foramina 

 transversaria, are usually absent. The vertebrarterial canal is 

 formed by the succession of the foramina transversaria on each 

 side, and contains the vertebral artery and vein. 



The thoracic vertebrae are distinguished by their very long 

 spinous processes and by the ribs which articulate with them. 

 Observe their zygapophyses. Thirteen pairs of ribs are present, 

 the dorsal portion of each being bone, the ventral portion being 

 cartilage. The nine anterior pairs of ribs articulate separately 

 with the sternum, and are called the true ribs ; the last three or 

 four pairs are united with one another ventrally, and are called 

 the false ribs; the ventral ends of the thirteenth pair are free, 

 and these are called the floating ribs. All the ribs except the 

 last two or three pairs articulate dorsally by two surfaces each ; 

 the capitulum, or head of the rib, articulates with the centrum 

 of two contiguous vertebrae, and the tuberculum articulates with 

 the transverse process. The tuberculum is absent on the last two 

 or three ribs. 



The sternum, or breastbone, is a row of bones which lies in the 

 ventral wall of the thorax and forms the ventral support of 

 the ribs. The foremost of these bones is the manubrium; the 

 hindermost is the ensiform process, the posterior part of which is 

 cartilaginous. 



The lumbar vertebrae are larger than the thoracic ; they are dis- 

 tinguished by the large size of the transverse processes, which 

 are directed forward and downward. 



The sacral vertebrae in the adult form a single bone called the 

 sacrum. The two innominate bones, which form the pelvis, are 

 closely joined with it. Note the two intervertebral foramina on 

 each side of it. 



The caudal vertebrae become gradually smaller toward the 

 hinder end, the last ones consisting of centra only. A num- 



