NUTRITION PROPER. 



upward {b and c) until the 

 edges meet above and are sol- 

 dered ; pt is the poison tube. 

 That this peculiar structure 

 was gradually formed is 

 shown by the fact that all 

 the steps of the process may ft 

 be found among living ser- 

 pents — viz., teeth slightly 

 grooved, deeply grooved, 

 and tubulated. All degrees 

 of virulence of the poison 

 may be found. The poison 

 gland is probably an extreme 

 modification of a salivary 

 gland, and the poison of 

 saliva. The saliva is slightly 

 poisonous in many animals. 



Observe again, third, that 

 the fang is formed in a fold 

 in the gum, and at first loose, 

 but afterward fixed to the jawbone. Also that in the 

 fold there are many subordinate folds, in each of which 



Fig. 192. — A longitudinal section 

 of the fang of a serpent : the 

 cross sections show mode of 

 formation of the poison tube ; 

 pt, poison tube ; tc, tooth cavity ; 

 pg, poison gland. 



on, 



Fig. 193.— Head of a rattlesnake, showing several fangs in different stages 

 of development. The arrow shows the poison tube ; m, maxillary bone. 



