22 ORGANIZATIONOFREPTILES. 



the animal: those respiring in water have gills, those breathing in atmospheric 

 air have lungs. The air enters through the trachea into the cells of the lungs, to 

 which the venous blood is carried, and where it undergoes great and important 

 changes, acquiring those properties that fit it for nutrition. "This function con- 

 nects the animal with the atmosphere by need of respiration, as nutrition connects 

 him with the earth; both are equally important conditions to the manifestation of 

 life; they both, though in different ways, contribute to maintain the constant change 

 of composition in the living body."* 



The lungs offer the most striking marks of distinction between the Reptiles, 

 Mammalia, and Birds. They are generally of great proportionate size, and are 

 composed of many large vesicles, which offer a strong contrast to the extremely 

 vascular and minute pulmonary cells of the Mammalia. This difference of structure 

 corresponds with the different wants of these animals. In the Mammalia and 

 Birds, it is necessary that all the blood of the body should pass to the puhnonary 

 cells to be exposed to the air, and in these the blood is more abundant; hence we 

 find the lungs composed of an infinitude of minute cells, where the pulmonary 

 artery terminates, and from which the radicles of the pulmonary veins originate. 

 In the Reptiles not more than one-third of the blood circulating in the system is 

 carried to the lungs, to undergo the change from venous to arterial; hence the 

 vessels spent on the lungs are far less numerous than in the higher classes, where 

 they form a beautiful net work. "This difference of structure, produces a difference 

 in the anunal, for as respiration imparts to the blood its warmth, its energy, this 

 will, in time, determine the degree of vigour in the animal functions; hence we 

 observe the great force of the powers of motion, the rapidity of digestion, the 

 violence of the passions, in Birds; hence the moderate degree of all these qualities 

 in the Mammalia; and hence again, the inertness, the inactivity, and apparent 

 stupidity of the other classes, as Reptiles and Fishes."t This structure of Reptiles 

 evinces less necessity for a constant and rapid change in their circulating fluids 



* Carus, Vergleich. Zoot. Zweit. Theil., p. 514. 

 t Cuvier, Legons d'Anat. Comp., torn. iv. p. 162. 



