272 



RESPIRATION. 



90 - 



In most aquatic animals, the respiratory organs have the form of 

 gills or branchiae; that is, filamentous prolongations of some part of the 

 integument or mucous membranes, which contain an abundant supply 

 of bloodvessels, and which hang out freely into the surrounding water. 

 In many kinds of amphibious reptiles, as, for example, in Menobranchus 



(Fig. 90), there are upon each 

 side of the neck feathery tufts or 

 prolongations from the mucous 

 membrane of the pharynx, which 

 pass out through lateral fissures 

 in the neck. Each filament con- 

 sists of a thin fold of mucous 

 membrane, containing in its in- 

 terior a network of minute blood- 

 vessels. The venous blood, as it 



enters the filament, is exposed to 

 HEAD AND G-ILLS OF MENOBRANCHUS. 



the influence of the surrounding 



water, and is thus converted into arterial blood. The apparatus is 

 further supplied with a cartilaginous framework and a set of muscles, 

 by which the gills are kept in motion, and constantly brought into con- 

 tact with fresh portions of the aerated fluid. 



In terrestrial and air-breathing animals, the respiratory apparatus is 

 situated internally. In salamanders and newts, for example, which, 

 though partly aquatic in their habits, are air-breathing animals, the 

 lungs are cylindrical sacs, running nearly the entire length of the body, 

 commencing anteriorly by a communication with the pharynx, and ter- 

 minating by rounded extremities at the posterior part of the abdomen. 

 These air-sacs have a smooth internal surface ; and 

 the blood which circulates through their vessels is 

 arterialized by exposure to the air contained in their 

 cavities. The air is forced into the lungs by a kind 

 of swallowing movement, and is after a time regur- 

 gitated and discharged, to make room for a fresh 

 supply. 



In frogs, turtles, and serpents, the cavity of the 

 lung, instead of being simple, is divided by incom- 

 plete partitions into a number of smaller cavities or 

 " cells." The cells all communicate with the central 

 pulmonary cavity ; and the partitions, which join 

 each other at various angles, are composed of thin, 

 projecting vascular folds of the lining membrane. 

 (Fig. 91.) By this arrangement, the extent of sur- 

 face presented to the air by the pulmonary membrane is increased, and 

 the arterialization of the blood takes place with a corresponding degree 

 of rapidity. 



In man, and in the warm-blooded quadrupeds, the lungs are constructed 

 on a plan essentially similar to the above, but which differs from it in 



Fig. 91. 



Li TING OF FROG, 



cut open, showing its 

 internal surface. 



