ORGANIC EVOLUTION 



187 



by a slender hollow stalk-like trachea, which divides into 

 two bronchial tubes, joining the right and left lungs. 

 By passing a pointed glass tube into the glottis and inflating 

 the lungs, their size, their constituent air cells and the com- 

 municating blood vessels in their wall may be clearly seen. 

 The circulatory system has for its central organ a heart of 



three chambers, two aur- 

 icles and a ventricle (fig. 

 115). The ventricle has 

 thick muscular walls, and 

 is the chief propelling 

 agent of the blood cur- 

 rent. It drives the blood 

 forward through the ar- 

 terial trunk, and out- 

 ward through the arches, 

 as indicated in the ac- 

 companying diagram. 

 The outward current is 

 called the arterial, the in- 

 ward, the venous circula- 

 tion. 



The carotid arch carries 

 blood anteriorly to the 

 head, the pulmo-cutan- 



eous inwardly to the lungs and externally to the skin 

 (whence its name), and the aortic arch carries the 

 greatest supply posteriorly and to peripheral parts of the 

 body, and distributes vessels through the mesentery to the 

 internal organs. 



The return currents reach the heart separately, entering 

 by the two auricles. That entering the left auricle is 

 returned from the lungs through the pulmonary veins. 

 That entering the right auricle (by way of the venous sinus, a 



FIG. 1 15. Diagram of the Amphibian heart, 

 and principal blood vessels. a, right 

 auricle; b, left auricle; c, ventricle; t, 

 arterial trunk; e. lung; d, liver; /, carotid 

 arch; g, aortic arch; h, pulmo-cutaneous 

 arch, with i. its cutaneous, and ;', its pul- 

 monary branch; k, pulmonary vein, with 

 the base of the corresponding vein from 

 the missing lung shown at I, m, the right 

 precaval ve ; n; H, postcava; 'o, anterior 

 abdominal vein and p, portal vein. 



