BEPTILIA. 551 



shoulder-joint of these reptiles, which are found to have a strong 

 ligament passing between the head of the humerus and the scapula, 

 exactly in the same manner as the ligamentum teres of the human 

 hip-joint. The use of such a deviation from the ordinary structure 

 of the articulation is obvious ; the frog, as it alights from those 

 long and vigorous leaps which form its ordinary mode of progres- 

 sion, receives the whole shock of its fall upon its fore-legs, and thus 

 this ligament becomes needful as an additional security to the ar- 

 ticulation in question. 



(604.) The skeleton of an Ophidian Reptile presents a strange 

 contrast to that of the Batrachian last described. Taking the Boa 

 Constrictor as an example of this order, we find the spine of this 

 enormous serpent composed of three hundred and four distinct 

 vertebrae, of which two hundred and fifty-two support ribs : flexi- 

 bility is, therefore, abundantly provided for in the construction of 

 these lithe and elegant beings, inasmuch as the division of their 

 spinal column into so many pieces allows the utmost pliancy in any 

 required direction. Flexibility, however, is not the only condition 

 requisite in this case ; strength and precision of movement are 

 equally indispensable, and the question is, how are these apparently 

 opposite qualities to be so combined and associated as not in the 

 slightest degree to interfere with each other. The mechanism con- 

 spicuous in the construction of the spine of a serpent is in this re- 

 spect truly admirable. The anterior extremity of the body of every 

 vertebra is rounded into a smooth and polished ball (Jig' 248, c), 

 which exactly fits into a hemispherical cup excavated in the sub- 

 stance of the vertebra next succeeding : a perfect ball-and-socket joint 

 is thus formed between every vertebra and that which precedes or 

 follows it; and thus the spine is rendered capable of the utmost lati- 

 tude of movement, and offers, at the same time, a firm purchase to 

 the muscles acting upon the vertebral column. To provide, how- 

 ever, against undue extent of motion in certain directions, we now 

 meet with other processes derived from the vertebral arches : in addi- 

 tion to those given Fig. 248. 

 merely as levers for 

 the attachment of 

 muscles,secondary 

 apophyses, called 

 oblique or articu- 

 lating processes, 

 become develop- 



