234 



FUNCTIONS OF NUTRITION. 



FIG. 51. 



membrane, containing a network of capillary blood-vessels. The appa- 

 ratus is supplied with a cartilaginous framework and a set of muscles, 

 by which the gills are kept in motion, and thus 

 brought in contact with fresh portions of the aerated 

 fluid. 



In terrestrial and air-breathing animals, the respi- 

 ratory apparatus is situated internally, under the 

 form of lungs. In salamanders and newts, the lungs 

 are cylindrical sacs, communicating anteriorly with 

 the pharynx, and terminating by rounded extremities 

 at the posterior part of the abdomen. The air, forced 

 into them from the pharynx, is after a time regurgi- 

 tated, to make room for a fresh supply. 



In frogs, turtles, and serpents, the lung is divided 

 by incomplete partitions into smaller cavities or 

 "cells." The cells all communicate with the cen- 

 tral pulmonary cavity ; and the partitions between them are vascular 

 folds of the lining membrane. (Fig. 51.) By this arrangement a 



FIG. 52. 



LUNG OF FROG, cut 

 open, showing its in- 

 ternal surface. 



HTJMAN LARYNX, TRACHEA, BRONCHI AND LUNGS; showing the ramifications of the bronchi, and 

 division of the lungs into lobules. 



greater extent of pulmonary surface is presented to the air, and the 

 aeration of the blood takes place with a corresponding rapidity. 



In the warm-blooded animals, the lungs are constructed on a plan 

 essentially similar to the above, differing from it only in the greater 



