THE RESPIRATORY SYSTEM. 3877 
and the air forced into the lungs by the elevation of the floor 
of the mouth. By a series of flank movements the elasticity 
of the lungs is aided in expelling the air through the now open 
nostrils. The respiration of the turtle and some other reptiles 
is somewhat similar. In the case of aquatic animals, both in- 
Fig. 312.—General view of air-reservoirs of duck, opened inferiorly : also the?r relations with 
principal viscera of trunk (after Sappey). 1, 1, anterior extremity of cervical reservoirs ; 
2, thoracic reservoir ; 3, anterior diaphragmatic reservoir ; 4, posterior ditto ; 5, abdom- 
inal reservoir: a, membrane forming anterior diaphragmatic reservoir; b, membrane 
forming posterier ditto ; 6, section of thoraco-abdominal diaphragm : cd, subpectoral pro- 
longation of thoracic reservoir ; e, pericardium ; f, f, liver ; g, gizzard ; h, intestines ; m, 
heart ; , n, section of great pectoral muscle above its insertion into the humerus ; 0, an- 
terior clavicle ; p, posterior clavicle of right side cut and turned outward. 
vertebrate and vertebrate, excepting mammals, the blood is 
freely exposed in the gills to oxygen dissolved in the water as 
it is to the same gas mixed with nitrogen in terrestrial animals. 
In the land-snail, land-crab, etc., we have a sort of intermedi- 
ate condition, the gills being kept moist. It is not to be for- 
