404 MANUAL OF ZOOLOGY. 



retracted 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. 157, 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 

 ball of the eye in front, is periodically shed with the rest of the 

 epidermis, the animal being rendered thereby blind for a few 

 days. The pupil of the eye is round in most Snakes, but forms 

 a vertical slit in the venomous Serpents and in the Boas. 



As regards the dental and maxillary apparatus of the 

 Serpents, the following points require notice. Firstly, in 

 consequence of the articulation of the lower jaw with a 

 movable quadrate bone, which is often directed backwards, 

 in consequence of the quadrate bone being connected with a 

 movable squamosal bone, and in consequence of the rami of 

 the jaw being united in front by ligaments and muscles only, 

 the mouth in the Snakes is capable of opening to an enormous 

 width, and the most astonishing feats in the way of swallowing 

 can be performed. Secondly, this structure of the jaws accords 

 exactly with the structure of the teeth, both concurring to 

 render the Snakes wholly incapable of anything like mastica- 

 tion, and at the same time capable of swallowing immense 

 morsels entire. The teeth, namely, are simply fitted for 

 seizing and holding the prey, but not in any way for dividing 

 or chewing it. In the harmless and most typical snakes, the 

 teeth are in the form of solid cones, which are arranged round 

 the margins of the upper and lower jaws, a double row existing 

 in the palate as well. Thirdly, in the venomous snakes, 

 however, the ordinary teeth are usually wanting upon the 

 superior maxillae, and these bones are themselves very much 

 reduced in size. In place of the ordinary teeth, they carry 

 the so-called poison-fangs (fig. 157, B). These are a pair of 

 long, conical, curved fangs, one on each maxilla, which can be 

 raised and depressed at will. Each tooth is perforated by a 

 tube, opening by a distinct aperture at the apex of the tooth, 



