xra PHYLUM CHORDATA 271 



elongated cavity, and its dorsal and ventral walls are raised up 

 into muscular ridges or trabeculae with interstices between them. 

 The conus springs from the right side of the base of the ventricle ; 

 it is separated from the latter by three small semilunar valves (v.), 

 and is traversed obliquely along its whole length by a large flap-like 

 longitudinal valve (Lv.) which springs from its dorsal wall and is free 

 ventrally. The conus passes without change of diameter into 

 a bulbus 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 longi- 

 tudinal 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 longitudinal valve (c f ). 



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. 942 

 and 943, car.) and lingual (Ig.) 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 with 

 one another above it to form the dorsal aorta (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 pul- 

 monary 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. 944, 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 



