VASCULAR SYSTEM. 289 



In the adult salamander the arrangement is shown in Fig. 162. 

 The ventral aorta has four branches (for the exact way in which 

 they come off see below). The gills have disappeared and the 

 afferent and efferent limbs of the vessels are continuous. The 

 first arch has become the carotid and has lost its dorsal connec- 

 tion with the second. The second arch has become the arch 

 of the aorta (systemic arch) ; the third remains as a small vessel 

 joining the second dorsally, but in some individuals it appears 

 to' be absent ; the fourth is continued as the pulmonary artery, 

 but retains its dorsal connection (ductus arteriosus or Botalli) 

 with the preceding arches 



In Triton the third arch has disappeared in the adult, and the first arch* 

 (carotid) retains its dorsal connection with the second. In Proteus and 

 Menobranchus the fourth vascular arch is not present in the adult, and- 

 the pulmonary is a branch of the third, the ventral end of which is con- 

 joined with that of the second. 



In Siren four arches are present and the arrangement is similar to that 

 in the larval salamander. In all the Perennibranchiates the relation of 

 the gills to the arches is similar to that found in the salamander larva. 



In the Anura * the third arch is completely absent in the adult, though^ 

 present in the larva. The pulmonary artery is a branch of the fourth,, 

 and the first and fourth arches are not usually connected with the dorsaH 

 system. The Carotid Gland is a plexus of small vessels inserted in the 

 course of the carotid artery. It is not derived from a gill as was formerly 

 supposed, but from the vessel which directly connects the efferent and 

 afferent limbs of the first branchial arch in the older larva. This vessel 

 becomes plexiform and together with some epithelial tissue derived from 

 the first branchial cleft gives rise to the carotid gland. 



Of the rest of the arterial system there is not much to be said. 

 The frog may be taken as typical, with one exception, viz. that 

 the visceral arteries are all gathered up into one, the coeliaco- 

 mesenteric, which leaves the left aortic arch close to its union 

 with the right. Almost all the blood of the left aortic arch of 

 the Anura goes into this vessel, the continuation of it to the 

 dorsal aorta being very small. In the Urodeles the visceral 

 arteries come off as many branches from the dorsal aorta. There 

 are two superior venae cavae (ductus Cuvieri) made up by the 

 junction of the jugulars (anterior cardinals) and the subclavians, 

 and an inferior vena cava which arises in the kidneys. The 

 venous blood of the hind end of the body is all sent either through 



* For an account of the development of the vascular arches of the frog, 

 see A. M. Marshall, " Vertebrate Embryology," London, 1893. 

 z n. u 



