COGSWELL ON THE HUMAN TEETH. 157 



The system of shedding and renewing is peculiar to the fish, also 

 in the Gangetic Crocodile called " Garrhiel," if at any time one of 

 the teeth be removed, it will be found hollow at its base, and partly 

 absorbed, presenting the appearance of a child's deciduous tooth, 

 when partly absorbed, and at the base of this, a new germ of the 

 auccessive tooth. The rodents also present this wonderful develop- 

 ment of tooth structure, where the canine teeth are wantine;. The 

 incisors two in number in each jaw are always growing, never 

 ceasing, while the animal lives, requiring constant exercise and 

 abrasion to maintain the normal form, and serviceable proportions, 

 as with the rat and rabbit. 



If in either of these an upper incisor is lost, or distorted union 

 of a broken jaw, rendering articulation imperfect, the incisors con- 

 tinue to grow, until they project like the tusks of an elephant, and 

 following their curve the points sometimes return to their head, 

 penetrate the skull, and cause death. I saw the skull of a beaver 

 in the Philadelphia Dental College Museum, the tooth of which 

 presented the above appearance, having grown out of all propor- 

 tion, causing no doubt the death of the animal. 



Higher in the scale we find in the genus Elephas the larger and 

 complex molars, never more than one or two partially in place and 

 use on each side, at any given time, for this progress of formation 

 and destruction, of shedding and replacement is constantly going 

 on, the molars or grinders succeeding one another from behind 

 forward. 



Another peculiar and complex condition of the dental system is 

 presented by the poisonous serpents, in which teeth are associated 

 with the tube or duct of a poison-bag and gland. 



These teeth are called poison-fangs ; they are confined to the 

 upper maxillary, only one (if more in number) contain the poison 

 fang, the tooth presents a curved appearance, on its sides are 

 grooves connecting with the poison-bag, and on the animal seizing 

 its prey the poison is forced into the wound along the groove in the 

 tooth, causing death as soon as the poison is diffused through the 

 system by absorption. 



Beside means of offence and defence in some animals the long 

 tusks &QVYQ as weapons of attack, and as instruments in aid of 



