CHAPTER XII. 

 Tab. XXIII. — nerves of the bill of a duck, valvulje 



CONNIVENTES. CHAP. XIII. AIR-BLADDER OF A FISH, AND 

 FANG OF THE VIPER. 



Fig. 1. The upper mandible of the duck, on which are dis- 

 tributed the first and second branches of tiie fifth pair of 

 nerves ; the former passing through the orbit to the extremi- 

 ty of the bill, and, together with the latter, supplying the 

 whole palatine surface. 



Fig. 2. A small portion of the human intestine cut open in 

 order to show the valvules connicentes. 



Fig. 3. The air-bladder in the roach. This vessel differs in 

 size and shape, in different species offish ; generally commu- 

 nicating, by one or more ducts, either with the oesophagas or 

 stomach ; by which means the fish receives or expels the air, 

 thus sinking or rising witliout effort : but as some are desti- 

 tute of this organ, it is considered as an accessary instrument 

 of motion. 



Fig. 4. The head of a viper of the natural size. 



Fig. 5. The fang magnified, at the root of which is the 

 gland which secretes the venom: a hair is represented in the 

 tube through which the poison is ejected. 



