OF THE OPHIDIANS IN GENERAL. 23 



possible volume, but of which the parts are susceptible of 

 an extraordinary enlargement, so as to permit serpents to 

 swallow the large animals intended by nature for their 

 sustenance. Now, to answer this end, the bony case of 

 the heads of serpents does not form, as in the greatest 

 number of the other vertebrata, an immoveable mass ; but 

 the component parts are so united together, that all of 

 them, except those enclosing the brain, are susceptible of 

 a greater or less movement, and generally in different 

 directions. This is particularly the case with the bones 

 which, entering into the formation of the lower jaw, give 

 configuration to the head. The development of the Tym- 

 panites, their mode of attachment, their mobility, wiiich 

 depend on not being fLxed to the cranium by their inferior 

 extremities ; in short, the structure of the lower jaw, the 

 two branches of which, instead of being united by a sym- 

 physis, are banded together by elastic ligaments, and are 

 thus susceptible of considerable separation ; these are the 

 circumstances which principally contribute to the enormous 

 enlargement of the mouths of serpents. The total want 

 of feet necessarily implies the absence of certain solid 

 parts, such as the sternum, the pelvis, &c., which unite 

 the luubs to the trunk ; the ribs free, and hence enjoying 

 an uniform mobility, contribute to the enlargement of the 

 intestinal cavity, and to that change of the form of the 

 trunk so visible in the different positions of the serpent in 

 running, in swimming, or in climbing. To obey these 

 various movements, the general integuments are divided 

 into numerous compartments, which form so many articu- 

 lations, parallel to the parts they cover ; the scales which 

 form the articulations, on the lower part of the animal, 

 are usually larger than the rest, and perform the office of 

 feet ; the ribs are attached to the lateral margin of the 

 internal face of these plates. The naked space of skin 

 between the scales is more considerable than in all other 

 reptiles ; and at the throat, this naked skin, in order to 

 accommodate itself to the separation of the jawbones, 

 occurs in the form of a longitudinal fosse, called the gular 

 fissure. By this structure of the general integuments, 

 these tunics, contracted in a state of repose, accommodate 



