270 



ELEMENTARY ANATOMY. 



[LESS. 



the mandibular symphysis, or line of junction of the two 

 jaws. 



FIG. 246. UPPER AND LOWER JAWS (SEEN FROM BEHIND) OF AN EAGLE RAY 

 (Afy/uafis) ) .ah<ywva% the elongated median dental plates and the hexagonal 

 lateral plates. 



One of the most singular modifications in the form of a 

 tooth is presented by the poison-fangs of Serpents. 



It is a common thing for a tooth to exhibit a vertical groove : 

 we see it even in the canines of Apes ; and in some extinct 

 forms (e.g. Ichthyosaurus] the teeth were furnished with a 

 number of such grooves. 



In poisonous Serpents, however^, the Rattlesnake one 

 large tooth on each side of the upper jaw has an exceedingly 

 deep, vertical, anterior groove, the margins of which groove 

 bend over till they meet and thus form a canal which opens 



FIG. 247. SIDE VIEW OF THE SKULL OF A RATTLESNAKE (Crotalus), showing 

 the large poison-fang implanted in the maxilla in front of the letters mx. 



a, articular hone of lower jaw ; bo, basi-occipital ; d, dentary; f, frontal ;f, 

 pre-frontal ; me, median ethmoid ; mx, maxilla ; 0, pro-otic ; /, parietal ;' pa, 

 para-sphenoid ; //, pterygoid ; px, pre-maxilla ; qn, quadrate ; qs, squamosal. 



widely above, but by a very small aperture below. It is down 

 this channel that the poison is poured, the tube leading from 

 the poison-gland opening into the upper part of the canal. 



