THE FROG. 191 



layer (epidermis) in it, which tends to peel off from the 

 thicker and vascular lower layer (dermis). 



Study of the Skeleton. With a prepared skeleton at 

 hand for reference, study the separate parts of a disarticu- 

 lated skeleton. 



First, note the arrangement of parts in the skeleton : 

 the central, vertebral axis, terminated in front by the 

 skull, and supported by two bony arches and two pairs 

 of jointed appendages. 



I. The Spinal Column. In the spinal column find ten 

 separate vertebrae, and a long, unsegmented terminal por- 

 tion (the urostyle). 



Note the presence of a double row of stout lateral pro- 

 cesses, and the absence of ribs. How many of the vertebrae 

 bear lateral processes ? Why are the lateral processes of 

 the third and ninth vertebrae stronger than the others ? 



Above the bases of the lateral processes note the articu- 

 lating processes (zygapophyses), and their method of 

 uniting adjacent vertebrae. 



On either side of the spinal column observe the row of 

 large openings through which the spinal nerves emerge. 

 What is the position of one of these openings in relation 

 to the vertebrae nearest it ? 



Isolate one of the midvertebrae, and in it see : 



The large, central neural canal. 



Ventral to this, the relatively small centrum (or body 

 of the vertebra), with its anterior face concave, and its 

 posterior face convex. 



Two stout pedicels of bone, arising dorsolaterally to 

 support the arch of the neural canal, and bearing (1) 

 the single pair of stout lateral processes, and (2) the two 

 pairs of articulating processes already noticed, and (3) on 

 the median dorsal line a short neural spine. Draw. 



Compare this vertebra with a typical vertebra of a fish. 



