328 
CHAPTER XII. 
Plate XX11L — nerves or the bill of a duck, valvule 
CONNIVENTES. CHAP. XIII. AIR-BLADDER OF A FISH, AND 
FANG OF THE VIPER. 
Fig. I, The upper mandible of the cluck, on which are distri¬ 
buted the first and second branches of the fifth pair of nerves; the 
former passing through the orbit to the extremity of the bill, and, 
together with the latter, supplying the whole palatine surface. 
This gustatory sensibility is the more necessary to those races of 
birds called palmipedes, such as penguins* the wild goose, ducks, 
&c. and the grallae, such as water-hens, curlews, woodcocks, &c. 
their sight being of no assistance to them in finding their prey in 
the mire. 
Fig. 2. A small portion of the human intestine cut open in 
order to show the valvule conniventes. It may be questioned, 
whether these extremely soft rugae or folds of the villous coat of 
the intestine can in the least retard the passage of the food through 
its canal; nor does the erect attitude of man require them; for, 
since there are as many of the convolutions of the intestines ascend¬ 
ing as there are descending, the weight of the food can have no in¬ 
fluence in the action of the intestine: it is certain, however, that 
this arrangement of the internal coat, affords a more extensive sur¬ 
face for the lacteals and secreting vessels; and this appeai-s to be the 
real use of the valvula conniventes. 
Fig. 3. The air-bladder in the roach. This vessel differs in size 
and shape, in different species of fish ; generally communicating, 
by one or more ducts, either with the cesophagas or stomach ; by 
which means the fish receives or expels the air, thus sinking or 
rising without effort: but as some are destitute of this organ, it is 
considered as an accessary instrument of motion. Such fish live 
almost uniformly at the bottom of the water. 
F ig. 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. 
Fig. 6, 7. See note, p. 126 
