xin PHYLUM CHORDATA 261 



pulmonary artery (pul.) to the lung, and a cutaneous artery (cu.) 

 to the skin. 



In the Tadpole there are four aortic arches, each consisting of 

 an afferent and an efferent branchial artery connected by the 

 capillaries of the gills. As the water-breathing larva undergoes 

 metamorphosis into the air-breathing adult the gills disappear ; 

 the first aortic arch loses its connection with the dorsal aorta and 

 beeomes the carotid trunk ; the second enlarges, retains its con- 

 nection with the dorsal aorta, and becomes the systemic trunk ; 

 the third disappears ; and the fourth sends off branches to the 

 lungs and skin, loses its connection with the dorsal aorta, and 

 becomes the pulmo-cutaneous trunk. 



The blood from each side of the head is returned by internal 

 (Fig. 880, int. ju.) and external (ext. ju.) jugular veins into the 

 precaval vein ( pr. v.\ which also receives the brachial vein (br.) from 

 the fore -limb, and the musculo-cutaneous vein (ins. cu.) from the skin 

 and muscles of the side and back, and part of the head. The two 

 precavals open separately into the sinus venosus. 



The course of the blood from the posterior parts of the body 

 is very different from what we have met with in Fishes, the 

 differences being due ^partly to the absence of a tail, partly to 

 a peculiar modification of the lateral, veins, and partly to the 

 substitution of the cardinals by a post-caval vein, found among 

 Fishes only in the Dipnoi. 



The blood from the front part of the hind leg is brought back 

 by & femoral vein (fm.) which, on reaching the coelome, divides into 

 two branches, a dorsal and a ventral. The dorsal branch is the 

 renal portal vein (rn. pt.) : it receives the sciatic vein (sc.) from the 

 back of the leg and passes to the kidney, when it breaks up into 

 capillaries. The ventral branch is the pelvic vein (pv.) : it unites 

 with its fellow of the opposite side to form the abdominal vein 

 (abd.) which passes forwards in the ventral body-wall, between the 

 linea alba and the peritoneum, to the level of the sternum, where 

 it turns inwards and divides into two branches, both breaking up 

 into capillaries in the liver. Just as it enters the liver it is joined 

 by the hepatic- portal vein (hp. pt.), bringing the blood from the 

 stomach, intestine, spleen, and pancreas. The abdominal vein also 

 receives vesical veins (ves.) from the urinary bladder, and a small 

 cardiac vein from the heart (cd\ It represents the lateral veins 

 of Elasmobranchs united in the middle ventral line: the pelvic 

 veins are their posterior free portions. 



The blood is collected from the kiditay^- by the renal veins (rn.), 

 which unite to form the large unpaired po^aval vein (pt. cv.). 

 This passes forwards through a notch in the liver, receives the 

 hepatic veins (hp.) from that organ, and finally opens into the sinus 

 venosus. Thus the blood from the hind limbs has to pass through 

 one of the two portal systems on its way back to the heart : part 

 of it goes by the renal portal veins to the kidneys, and thence by 



