78 KESriRATORY SYSTEM OF WARM-BLOODED ANIMALS. 



adduced to show that the consumption of oxygen, and - the corresponding waste of 

 the tissues, corresponds exactly with the development, habits and temperature of 

 animals. The viscera of this animal, reduced to one-half their diameter, are repre- 

 sented in Fig. 20. 



The lungs of the several orders of reptiles are formed upon one type, being capa- 

 cious sacs, whose walls are divided into sacculi, and supplied with bloodvessels 

 according to the perfection of the organs and apparatus, and the habits of the 

 animal. 



From the internal surface membranous septa project inwards, dividing the inte- 

 rior of the organ into numerous polygonal cells, which are themselves subdivided 

 into smaller compartments. The bloodvessels are distributed over the internal 

 walls of the lungs and over the sides of the pulmonary cells. 



In serpents one lung only is developed, and the pulmonary cells are most nume- 

 rous in the superior portion, whilst the inferior part of the long cylindrical lung is a 

 mere menabranous sac with few or no bloodvessels ramifying upon its walls. (See 

 Figure 21 .) 



We find the greatest number of the polygonal cells and the greatest distribution 

 of the bloodvessels in the pulmonary organs of the Clielonia and Sauria, thus fore- 

 shadowing the condition of the lungs in birds and mammals. 



In these orders the lungs are filled, more or less, by a coarse and fine network 

 or areolar tissue, forming angular or rounded meshes, which rest partly upon the 

 walls of the lungs, and enclose lesser meshes or air-cells. The bloodvessels ramify 

 over the meshes as well as over the walls of the lungs. The sacculi thus formed 

 communicate with each other, and can all be inflated from any one point. 



The size of the lungs differs in the different orders according to their structure 

 and habits. Amongst the Chelonians, we find the most capacious lung in the 

 gopher [Testudo Polijphemus). These animals burrow deeply in the ground, and 

 need large lungs as a reservoir of air. In aquatic serpents which remain under 

 the water for a great length of time, the lungs are capable of holding a greater 

 quantity than those of land serpents. 



In mammals and birds the blood is abundant, and the circulation rapid, and the 

 wastes and metamorphoses of the tissues correspondingly great, and the lungs 

 are composed of an infinitude of minute cells containing air, and surrounded by a 

 capillary network. 



The respiratory system of birds is far more highly developed than that of reptiles, 

 but not so concentrated as that of the mammals. In this class the lungs are no 

 longer closed bags like those of reptiles, but are spongy masses of great vascularity 

 communicating with numerous air-sacs and the cavities of the bones. The main 

 trunks of the bronchial tubes pass through the lungs and open into the cavity of 

 the thorax. The whole thoracico-abdominal cavity is divided by bands of serous 

 membrane into numerous cells communicating with each other and the cavities of 

 the hollow and spongy bones. 



In many birds, especially those of powerful flight, the air is admitted into the 

 interspaces between the muscles, and between the skin and muscular system. By 

 this arrangement, which reminds us of the tracheal system of insects, the air pene- 



