176 VERTEBRATES. 



lies a large artery. A similar pair of processes arise, and 

 unite in a. similar w.ay on the ventral surface, to form a 

 long hemal spine. Another large artery is lodged in the 

 arch at the base of this spine. Notice that both spines 

 are directed backward. Find four small processes on the 

 anterior margin, and four on the posterior margin, of 

 the centrum. These are articulating processes (zyga- 

 pophyses). They join corresponding processes on adja- 

 cent vertebrae, and serve to hold the several vertebrae 

 more firmly together. Study the relation between these 

 processes in vertebrae that have not been disarticulated. 



Draw a vertebra as seen from one side, and as seen from 

 one end. 



II. Ribs and Interneurals. Compare this vertebra 

 with others both before and behind it in the tail region. 

 Observe that in several of the foremost vertebrae of this 

 region the large processes of the ventral side do not unite 

 to form a single hemal spine, but, diverging, form two 

 spines. Proceeding forward into the body region, ob- 

 serve that there they no longer form a hemal arch, 

 that they become more and more divergent, and that 

 ribs are added to their outer ends. Observe that the 

 articulation of each rib is such that it appears to be 

 spliced to the process which supports it. How many 

 pairs of ribs are there? Observe that the neural spines 

 of the rib-bearing vertebrae are reduced in length and 

 modified in form to accommodate the bones which sup- 

 port the anterior dorsal fin. The latter bones, because 

 they usually fit in between the neural spines, are called 

 interneurals. 



III. Spines and their Supports. The part of the spinal 

 column anterior to the ribs consists of several vertebrae 

 solidly welded together, and so greatly modified as 

 scarcely to be recognized as vertebrae at all. They are 

 joined to the long dorsal process from the skull to the 



