SCALED REPTILES—SNAKES. 413 



aid of limbs ; but it is probably less widely known that certain members of 

 the group retain vestiges of the hind-legs in the shape of two spur-hke pro- 

 jections from the body in the neighbourhood of the vent. And in such 

 forms, if the skeleton be examined, it will be found that there are remnants 

 of the bony framework of the pelvis and upper part of the hind-limbs. All 

 these facts clearly show that snakes trace their descent from reptiles endowed 

 with four complete limbs. Under the heading of the Lacertilia it has been 

 mentioned that the two branches of the lower jaw of a snake are united at the 

 chin by a ligament, so as to be capable of wide separation from one another ; 

 and in many members of the group the capacity of the mouth can be further 

 augmented by a similar arrangement in the upper jaw, the bones of both the 

 jaws themselves, and likewise those of the palate, being movably joined to- 

 gether. This arrangement permits these reptiles to devour a prey of con- 

 siderably larger size than the normal calibre of their mouth and throat. In 

 place of movable eyelids, snakes have a continuous transparent skin stretched 

 over the eyes, which is periodically oast with the slough ; and in iio case is 

 there any external opening to the ears. A peculiar feature in a serpent is 

 the enormous number of vertebrae contained in the backbone, not less re- 

 markable being the number of pairs of ribs. In fact, from the head to a long 

 way down the tail, each joint of the backbone carries a pair of these append- 

 ages, which increase gradually in length from the head to the fore-part of the 

 trunk, and thence as gradually diminish to the tail. There being no vestiges 

 of collar-bones or breast-bone, each rib terminates below in a freo point. 



In order to give extra flexibility, and at the same time strength, to this 

 exceedingly elongated backbone, the vertebrae of serpents are provided 

 with extra articulations, a wedge-like projection from one fitting in to a 

 corresponding hollow in the next. Such additional articulations are, however, 

 by no means confined to snakes, but also occur in some lizards, notably the 

 monitors. In ordinary land vertebrates the ribs are largely connected with 

 the function of breathing ; but in snakes — in addition to supporting the walls 

 of the trunk, and thus keeping open the cavity of the chest — their chief 

 function is in progression, so that these reptiles may really be described as 

 rib-walkers. On the under surface of the body the majority of snakes have 

 a series of large transverse horny shields, which are much wider than long, 

 and in the trunk-region extend right across the lower surface ; these shields, 

 it may be remarked, being absent in the limbless lizards. In the tail the 

 enlarged shields not unfrequently form a double longitudinal series. Each 

 of the complete transverse shields corresponds to the termination of a pair of 

 ribs. By holding on to the inequalities of the surface they are traversing by 

 the free edges of the shields — which project backwards — and then by drawing 

 closer together the ribs of one side of the body, and afterwards those of the 

 other, a snake produces the well-known lateral undulations of its body. By 

 straightening out the front part of the body when a firm hold is obtained, 

 and then drawing after it the hinder portion, progression is eifected. On a 

 smooth surface movement is impossible, and no snake ever produces vertical 

 undulations of its body. All snakes have a prehensile forked tongue, capable 

 of retraction within a basal sheath, and during their waking hours kept in 

 constant movement. Usually the head of a snake, both above and below, is 

 invested with a number of large and symmetrically-arranged polished horny 

 shields, to each of which a distinctive name is given. Internally, snakes are 

 remarkable for the great elongation of the lungs, or rather lung, since, as a 

 rule, only the one on the right side is functional. 



