MANUAL OF ZOOLOGY. 



ward evidence of their existence beyond the occasional presence 

 of short horny claws or spurs ("calcaria"). In the entire ab- 

 sence, then, or rudimentary condition of the limbs, the Snakes 

 progress by means of the ribs. These bones are always ex- 

 tremely numerous (sometimes amounting to more than three 

 hundred pairs), and in the absence of a sternum, they are, of 

 course, extremely movable. Their free extremities, in fact, are 

 simply terminated by tapering cartilages, which are attached 

 by muscular connections to the abdominal scales or " scuta " 

 of the integument. By means of this arrangement the Ser- 

 pents are enabled to progress rapidly, walking, so to speak, 

 upon the ends of their ribs ; their movements being much facili- 

 tated by the extreme mobility of the whole vertebral column, 

 conditioned by the cup-and-ball articulation of the bodies of 

 the vertebrae with one another. 



The body in the Snakes is covered with numerous scales, 

 developed in the dermis, and covered by a thin, translucent, 

 superficial epidermic pellicle, which is periodically cast off and 

 renewed. Usually the scales are flat and overlap one another ; 

 but sometimes they are tubercular and do not overlap. On 

 the head and along the abdomen these scales are ^larger than 

 over the rest of the body, and they constitute what" are known 

 as the " scuta " or shields. 



The only other points in the anatomy of the Ophidia which 

 demand special attention are the structure of the tongue, teeth, 

 and eye. 



The tongue in the Snakes is probably an organ more of 

 touch than of taste. It consists of two muscular cylinders, 

 united towards their bases, but free towards their extremities. 

 The bifid organ, thus constituted, can be protruded and re- 

 tracted at will, being in constant vibration when protruded, 

 and being in great part concealed by a sheath when retracted. 



As regards the eye of Serpents (fig. 296, A) the chief 

 peculiarity lies in the manner in which it is protected exter- 

 nally. There are no eyelids, and hence the stony unwinking 

 stare of all snakes. In place of eyelids, the eye is surrounded 

 by a circle of scales (e e), to the circumference of which is 

 attached a layer of transparent epidermis, which covers the 

 whole eye (//), and is termed the antocular membrane. This 

 is covered internally by a thin layer of the conjunctiva, which 

 is reflected forwards from the conjunctiva covering the ball of 

 the eye itself. In this way a cavity or chamber is formed 

 between the two layers of conjunctiva, and the lachrymal 

 secretion, by which the eye is moistened, is received into this. 

 The outer epidermic layer (antocular membrane) covering the 



