THE SPINAL COLUIIN OF REPTILES. 1 67 



becomes united to the end of the vertebra 

 in advance of it, continuing to play in the 

 cup of the next succeeding. Subsequently, 

 this cartilaginous ball becomes ossified, and 

 part and parcel of the vertebra to which 

 it was attached. Thus a true ball-and-socket 

 joint is formed — the projecting ball or con- 

 vexity at the end of one vertebra playing in 

 the concavity at the anterior portion of the 

 next vertebra in succession. 



This description refers to the ten true 

 vertebrae of which the spine of the frog 

 consists ; but below these, and united to 

 them at an angle which produces an abrupt 

 point in the back of the frog, as if the 

 spine were broken, runs a single slender bone, 

 originally divided, being, in fact, a consoli- 

 dation of a few of the most basal tail-bones 

 of the tadpole, and those immediately an- 

 terior to its base: this bone is called the os 

 coccygis, and on each side of it, but removed 

 from it, run the slender haunch-bones. The 

 breast-bone in the frog, though partially carti- 

 laginous, gives origin to the abdominal muscles, 

 and is considerably developed ; the shoulder- 

 bones consist of the clavicle, the coracoid 

 bone, and the scapula, or blade-bone ; and 



