266 VERTEBRATE ANIMALS, 
pupil is round in most serpents, but it forms a vertical slit or 
fissure in the venomous snakes and in the Boas. 
As regards the teeth, it is to be noticed that the snakes are 
not in the habit of chewing their prey, but of swallowing it 
whole, and the construction of their dental apparatus is in 
accordance with this peculiarity. The lower jaw, as before 
said, articulates with the skull by means of a quadrate bone (fig. 
138), and this in turn is movably jointed to the cranium. The 
two halves of the lower jaw are also merely united loosely in 
front by ligaments and muscles. In consequence of this peculiar 
arrangement of parts, the serpents have the power of opening 
the mouth to an extraordinary width, and they can perform the 
most astonishing feats in the way of swallowing. The teeth are 
simply fitted for seizing and.holding the prey, but not in any 
Fig. 143.-—A, Diagrammatic Section of theiEye of a Viper (after Cloquet).: a Eye- 
ball ; 4 Optic nerve ; ¢ Chamber into which the tears are poured ; d Epidermic 
layer covering the eye. B, Head of the common Viper (after Bell), showing the 
poison-fangs. 
way for chewing or dividing it. In the harmless snakes the 
teeth are in the form of solid cones, which are arranged in rows 
round the whole of the upper and lower jaws, a double row 
existing on the palate as well. In the venomous snakes, on the 
other hand, the ordinary teeth are usually wanting upon the 
upper jaws, and these bones are themselves much reduced in 
size. In place of the ordinary teeth, however, the upper jaws 
carry the so-called “ poison-fangs” (fig. 143, B). These are a 
pair of long curved fangs, one on each maxilla or upper jaw- 
bone, which, when not in use, are pointed backwards, but can 
be raised at will by special muscles, by the elevation of the 
movable maxillz. Each tooth is perforated by a fine canal or 
tube, which opens by a distinct aperture at the point of the 
