CIRCULATORY ORGANS. 



299 



abdominal vein (fig. 294), while the blood from the hind limbs may return to the 

 heart through either the anterior abdominal or the renal portal system. 



In the lungless salamanders (p. 258) the heart and blood-vessels show corre- 

 sponding modifications. There is no septum atriorum and the pulmonary arteries 

 and veins fail to develop. The cutaneus arteries and the smaller vessels supplying 

 the pharyngeal region are greatly enlarged, respiration taking place through the 

 skin and the mucous membrane of the throat. 



The action of the anuran heart may be out- 

 lined here. The two atria contract at the same 

 time, forcing arterial and venous blood into the 

 ventricle, but it is kept from mixing by the mus- 

 cular bands already alluded to. At the systole 

 of the ventricle the venous blood, which is near- 

 est the truncus, is first forced forward. This 

 takes the most direct course through the wide 

 and shorter pulmonary arteries, which are prac- 



FIG. 304. FIG. 305. 



FIG. 304. Diagram of the aortic arches in amphibia. Arterial blood cross lined, 

 venous black. The gill circulation omitted, its course indicated by arrows; the permanent 

 circulation after the absorption of gills shown, eg, carotid gland; da, dorsal aorta; d, 

 ductus Botalli; pa, pulmonary artery; va, ventral aorta; 3-6, aortic arches. 



FIG. 305. Heart of snapping turtle, Chelydra serpentina (Princeton, 479). aa, aortic 

 arch; c, cceliac artery; da, dorsal aorta; db, Botall's duct; ec, ic, external and internal 

 carotids; la, left auricle; p, pulmonary artery; ra, right auricle; sc, subclavian artery; v, 

 ventricle; m, mesenteric artery. 



tically empty at the time. The next portion of the blood, containing both arterial 

 and venous, follows the next easiest course through the aortic arches, while the 

 last to leave the ventricle, consisting of pure arterial blood, can only go into the 

 carotids, where the resistance is greater on account of the small size of the vessels 

 and the obstacles presented by the carotid glands. 



REPTILES. In the reptiles the division of the heart (fig. 287) is carried still 

 farther and the sinus venosus tends to be merged in the right atrium. The atrial 

 septum is complete and is continued forward as a ventricular septum, partially 



