120 THE VITAL FUNCTIONS. 



low cones, composed of ivory and enamel, are renewed by 

 the new tooth (as is shown at A, in Fig. 284,) being formed 

 in the cavity of the one (B) which it is 

 to replace, and not being enclosed in any 

 separate cavity of the jaw bone (c.) As 

 this new tooth increases in size, it press- 

 es against the base of the old one, and 

 entering its cavity, acquires the same 

 conical form; so that when the latter is 

 shed, it is already in its place, and fit 

 for immediate use. This succession of 

 teeth takes place several times during 

 the life of the animal, so that they are sharp and perfect at 

 all ages. 



The fangs of serpents are furnished, like the stings of 

 nettles, with a receptacle at their base for a poisonous li- 

 quor, which is squeezed out by the pressure of the tooth, at 

 the moment it inflicts the wound, and conducted along a 

 canal, opening near the extremity of the tooth. Each fang 

 is lodged in a strong bony socket, and is, by the interven- 

 tion of a connecting bone, pressed forwards whenever the 

 jaw is opened sufficiently wide; and the fang is thus made 

 to assume an erect position. As these sharp teeth are very 

 liable to accidents, others are ready to supply their places 

 when wanted: for which purpose there are commonly pro- 

 vided two or three half-grown fangs, which are connected 

 only by soft parts with the jaw, and are successively moved 

 forwards into the socket to replace those that were lost.* 



The tube through which the poison flows is formed by 

 the folding in of the edges of a deep longitudinal groove, 

 extending along the greater part of the tooth; an interval 

 being left between these edges, both at the base and extre- 

 mity of the fang, by which means there remain apertures at 

 both ends for the passage of the fluid poison. This struc- 

 ture was discovered by Mr. T. Smith in the Coluber naia> 



* Home, Lectures, &c. 1. 333. 



