DIGESTIVE ORGANS. 213 



(acrodont); applied to their inner side (pleurodont, fig. 97, d}\ or 

 have their roots implanted in grooves or sockets or alveoli (thecodont). 

 Mention must also be made of the poison fangs of certain serpents. 

 These are specialized teeth borne on the maxillary bones and are either 

 permanently erect (proteroglypha) or the bone may turn, as on a pivot, 

 so that when the mouth is closed the teeth lie along the roof of the 

 mouth, but when it is opened, they are brought into position for striking 

 the prey (vipers, rattlesnakes solenoglypha). Correlated with the 

 fixed or movable condition is a modification in the teeth themselves. In 

 the proteroglypha a groove runs along the anterior side of the fang by 



FIG. 215. Poison gland and fang of rattlesnake, Crotalus horridus. (Princeton 1404) 



p, poison gland; /, labial glands. 



which the poison is conducted from the poison gland into the wound. 

 In the solenoglypha the groove is rolled into a tube with openings near 

 the base and apex of the tooth (fig. 215). In these solenoglyphous 

 snakes only a pair of fangs are functional at a time, but there are 

 reserve teeth which can come into use on the loss of the first. 



The greatest variation is found in the teeth of mammals, the heter- 

 odont dentition being the rule. Four kinds of teeth are recognized. 

 These are the incisors in the premaxillary bones, followed by a single 

 canine at the anterior end of each maxillary bone. This resembles 

 the incisors and differs from the other maxillary teeth in its conical shape 

 and single root. Behind the canines come the premolars (the bicus- 

 pids of the dentists) which have two roots and complicated crowns and 

 appear in both milk and permanent dentitions. Lastly are the molars, 

 like the premolars in form, with several roots, but appearing only in the 

 permanent dentition. The corresponding teeth in the lower jaw have 

 the same names. 



In a few mammals, like the whales, all of the teeth are of a simple 

 conical shape, but in the majority the crown of the molars is marked 

 by projections cones, tubercles, crests, etc. which are variously 



