210 OPHIDIA. 



(fig. 637). The mental scutes i.e., the scutes in the mental groove 

 on the ventral surface between the rami of the lower jaw (fig. 637, q) 

 may be mentioned as peculiar to most snakes ; in front of these 

 two accessory labial scutes on either side form with the median 

 labial scute (o) the anterior boundary of the mental groove. The 

 scutes on the abdomen are for the most part broad, and invest the 

 trunk like transverse bands (fig. 637 b, t) ; but scales and small 

 median scutes may also be present here. The ventral surface of the 

 tail, on the other hand, is, as a rule, covered by a double, or rarely 

 by a single, row of scutes. Snakes moult several times in the course 

 of the year : they strip off the whole of the epidermis on which the 

 sculpture of the cutis is repeated. 



The internal organisation corresponds with the requirements of 

 the elongated form of the body, as well as with the mode of locomo- 

 tion and nourishment. A long and extensible gullet with thin walls 

 leads into the dilated saccular stomach, which is followed by a rela- 

 tively short small intestine. The larynx is placed extraordinarily 

 far forward, and can be projected into the mouth during the long 

 and difficult act of swallowing. The trachea is extremely long, and 

 often contains respiratory air-cells in its course. The left lung is 

 usually entirely rudimentary, while the right lung is correspondingly 

 large, and is transformed at its posterior end into a vesicular air- 

 reservoir. 



The auditory organ is without an apparatus for conducting sound, 

 and the eyes have no movable lids. The eye-ball, with its usually 

 vertical pupil, is covered by the skin, which is here transparent, and 

 behind which it is bathed by the lacrymal fluid. The nasal apertures 

 are usually placed quite at the apex or on the lateral margins of the 

 snout. The forked horny tongue serves not as an organ of taste, 

 but as a tactile organ, and is enclosed in a sheath, from which it can 

 be protruded through an indentation of the extremity of the snout, 

 even when the mouth is closed. 



The Snakes move principally by means of lateral flexions of the 

 vertebral column. The vertebrae are very numerous, and almost 

 always bear ribs in the region of the trunk. The centra are concave 

 in front and convex behind ; they are connected with one another 

 by free ball and socket joints, and by horizontal articular surfaces of 

 the transverse processes in such a manner that dorso-ventral move- 

 ments are impossible. The ribs are also freely articulated with the 

 vertebral bodies, and can be moved backwards and forwards, move- 

 ments which are of cn'eat use in assisting locomotion. The Snakes 



