146 



THE VITAL FUNCTIONS. 



284 



rows of teeth are lodged in each jaw ; but only one 

 of these rows projects and is in use at the same 

 time ; the rest lying flat, but ready to rise in order 

 to replace those which have been broken or worn 

 down. In some fishes the teeth advance in propor- 

 tion as the jaw lengthens, and as the fore teeth are 

 worn away : in other cases they rise from the sub- 

 stance of the jaw, which presents on its surface an 

 assemblage of teeth in different stages of growth ; 

 so that in this class of animals the greatest variety 

 occurs in the mode of the succession of the teeth. 



The teeth of the Crocodile, which are sharp- 

 pointed hollow cones, composed of ivory and ena- 

 mel, are renewed by the new tooth (as is shown at 



A, in Fig. 284), being formed in 

 the cavity of the one (b) Mhich it 

 is to replace, and not being in- 

 closed in any separate cavity of 

 the jaw bone (c). As this new 

 tooth increases in size, it presses 

 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 with a recep- 

 tacle at their base for a poisonous liquor, 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 



