A REPRESENTATIVE VERTEBRATE. FROG 



337 



liver; and the anterior mesenteric, which is 

 distributed to the intestine, spleen, and 

 cloaca. Posterior to the origin of the 

 coeliacomesenteric, the dorsal aorta gives off 

 several renal arteries which supply the kid- 

 neys. A small posterior mesenteric artery 

 arises near the posterior end of the dorsal 

 aorta and passes to the large intestine; and 

 in the female, to the uterus. The dorsal aorta 

 finally divides into two common iliac ar- 

 teries, which are distributed to the ventral 

 body wall, the rectum, bladder, the anterior 

 part of the thigh (femoral artery), and 

 other parts of the hindlimbs (sciatic artery). 



The veins (Fig. 221) return blood to the 

 heart. The blood from the lungs is collected 

 in the pulmonary veins and poured into the 

 left atrium. Venous blood is carried to the 

 sinus venosus by three large trunks: the two 

 anterior venae cavae and the posterior vena 

 cava. The anterior vanae cavae receive blood 

 from the ( 1 ) external jugulars, which collect 

 blood from the tongue, thyroid, and neigh- 

 boring parts; (2) the innominates, which 

 collect blood from the head by means of 

 the internal jugulars and from the shoulder 

 by means of the subscapulars; and (3) the 

 subclavians, which collect blood from the 

 forelimbs by means of the brachial, and 

 from the side of the body and head by 

 means of the musculocutaneous veins. The 

 posterior vena cava receives blood from the 

 kidneys by means of 4 to 6 pairs of renal 

 veins; from the reproductive organs by 

 means of spermatic or ovarian veins; and 

 from the liver by means of two hepatic 

 veins. 



The veins which carry blood to the kid- 

 neys constitute the renal portal system. The 

 renal portal vein receives the blood from the 

 hindlimbs by means of the sciatic and 

 femoral veins, and from the body wall by 

 means of the dorsolumbar vein. While it is 

 true that the blood systems of the various 

 vertebrates are built on the same general 

 plan, there is no renal portal system in 

 man. 



The liver receives blood from the hepatic 



portal system. The femoral veins from the 

 hindlimbs divide, and their ventral branches 

 unite to form the ventral abdominal vein. 

 The ventral abdominal vein collects blood 

 from the bladder, ventral body wall, and 

 heart. The hepatic portal vein carries blood 

 into the liver from the stomach, intestine, 

 spleen, and pancreas. This passage of the 

 venous blood from the intestinal tract 

 through the liver before entering the main 

 circulation makes it possible for the liver 

 to add or remove substances from the blood 

 as physiologic needs may require. 



Circulation in the frog takes place in the 

 following manner: the sinus venosis con- 

 tracts, forcing the nonoxygenated venous 

 blood into the right atrium (Fig. 220). 

 Oxygenated blood from the lungs passes into 

 the left atrium. Then both atria contract 

 and force their contents into the ventricle. 

 Formerly, it was thought that when the 

 ventricle contracted, the spiral valve de- 

 flected nonoxygenated blood from the right 

 side into the pulmocutaneous arch and oxy- 

 genated blood from the left side into the 

 carotid and aortic arches. But experiments 

 have proved that the two blood streams mix. 

 Therefore, it must be assumed that mixed 

 blood is pumped to all parts of the frog's 

 body. The blood is prevented from flowing 

 back into the heart by means of valves (Figs. 

 220 and 384). Respiration through the skin 

 of the frog, both in water and on land, is 

 thought to compensate, at least in part, for 

 failure of all nonoxygenated blood to be 

 pumped to the lungs. 



The blood that is thus forced through 

 the arteries makes its way into tubular blood 

 vessels that become smaller and smaller until 

 the extremely narrow capillaries are reached 

 (Fig. 383, p. 529). Here food and oxvgen 

 are delivered to the tissues, and waste prod- 

 ucts are taken up from the tissues. The 

 renal portal system carries blood to the 

 kidneys, where urea and similar impurities 

 are taken out. The hepatic portal system 

 carries blood to the liver, where bile and 

 glycogen are formed. The blood brought to 



