sect, xii PHYLUM CHORDATA 379 



chial vessels run inwards and backwards from the loops 

 under cover of the mucous membrane of the roof of the 

 mouth to unite in a large median trunk — the dorsal aorta 

 (d.ao). From the first efferent vessel, that from the first 

 or hyoidean gill, arises the carotid artery, which runs for- 

 wards and bifurcates to form the internal and external 

 carotid arteries (c.a), supplying the head with arterial 

 blood. 



The dorsal aorta runs backwards throughout the length 

 of the body- cavity, giving off numerous branches, and is 

 continued as the caudal artery (cd.a), which runs in the 

 canal enclosed by the inferior arches of the caudal vertebrae. 



The veins are very thin-walled, and the larger trunks are 

 remarkable for their dilated character, from which they 

 have obtained the name of sinuses, though they are true 

 vessels and not sinuses in the sense in which the word is 

 used in dealing with the Invertebrates. 



The venous blood is brought back from the head by a 

 pair of jugular or anterior cardinal sinuses (j. v) , and from 

 the trunk by a pair of posterior cardinal sinuses (crd.v). At 

 the level of the sinus venosus the anterior and posterior car- 

 dinals of each side unite to form a short, nearly transverse 

 sinus, the precaval sinus or ductiis Cuvieri ( pr. cv. v) which 

 is continued into the lateral extremity of the sinus venosus. 



There are two portal systems of veins, the renal portal 

 and the hepatic portal {h.p.v), by which the kidneys and 

 liver, respectively, are supplied with venous blood. The cau- 

 dal vein (cd.v), which brings back the blood from the tail, 

 running, along with the caudal artery, through the inferior 

 arches of the vertebrae, divides on entering the abdominal 

 cavity into right and left renal portal veins (r.p.v), which 

 end in a number of afferent renal veins supplying the kidneys. 



The hepatic portal vein (h.p.v) is formed by the conflu- 



