68 



Basic Structure of Vertebrates 



Fig. 69. The lymphatics of the scrotum, showing the transition of the capillaries 

 to the vessels with valves (a, a, a). (After Teichmann. Courtesy, Jackson: "Morris' 

 Human Anatomy," Philadelphia, The Blakiston Company.) 



arteries pass to the head, and a subclavian artery (sometimes con- 

 nected to the dorsal aorta) to each pectoral fin or leg. The dorsal aorta 

 gives off branches to the viscera — a celiac artery to the stomach and 

 liver, two or more mesenteric arteries to the intestine, and pairs of 

 ovarian, spermatic, and renal arteries. Segmentally arranged pairs 

 of parietal arteries pass to the muscles of the body-wall, a pair of 

 iliac arteries supply the pelvic fins or legs, and, posterior to the 

 trunk, the aorta continues as the caudal artery (Figs. 64, Squalus; 

 71A). 



In lung-breathers without gills (Fig. 71D, E), the blood- 

 stream is "shunted" off from the main course of the circulation to pass 

 through the respiratory capillaries of the lungs. In connection with this 

 arrangement, the heart becomes more or less completely divided and 

 the blood passes through the heart twice in each of its trips around the 

 body. A median partition divides the primitively single auricle into 

 right and left auricles and in reptiles, birds, and mammals, the 

 ventricle is similarly divided into right and left chambers, although 

 the division is incomplete in most reptiles (Fig. 72). The "impure" 

 blood returning from the general (systemic) circulation passes through 

 the right chambers of the heart, and the right ventricle pumps it to the 

 lungs via the pulmonary arteries. Returning from the lungs, pul- 



