280 ZOOLOGY SECT. 



ventrally. The conus passes without change of diameter into 

 a lulbus aortce, the two being separated by a semilunar valve (v!) 

 and by the free end of the longitudinal valve. The bulbus 

 gives off two branches, right and left, each of them divided by 

 two longitudinal partitions into three vessels, an inner or anterior, 

 the carotid trunk (car. tr.), a middle, the systemic trunk or 

 aortic arch, and an outer or posterior, the pulmo-cutaneous trunk 

 (pul. cu. tr.). The carotid and systemic trunks communicate 

 separately with the bulbus ; the two pulmo-cutaneous trunks 

 communicate with the anterior end of the conus by a single 

 valvular aperture placed just behind the free end of the longi- 

 tudinal valve (c/). 



After being bound together in the way described for a short 

 distance, the carotid, systemic, and pulmo-cutaneous trunks separate 

 from one another. The carotid trunk divides into carotid (Figs. 931 

 and 932, car.) and lingual (lg.) arteries for the supply of the head, the 

 former having at its base a small swelling, the carotid " gland " or 

 labyrinth (car. gl.) with a spongy interior containing numerous cavities. 

 The systemic trunks curve round the gullet and unite witli one 

 another above it to form the dorsal awta (d. ao.\ from which, or 

 from one of the systemic trunks themselves, the arteries to all 

 parts of the body, except the head, the lungs, and the skin, 

 are given off. The pulmo-cutaneous trunk divides into two, a 

 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 

 becomes 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. 933, 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 (ms. 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 part 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 

 replacement of the cardinals by a postcaval vein, found among 

 Fishes only in the Dipnoi. 



