THE VASCULAR SYSTEM 197 



to nearly the whole of the body except the brain, the lungs, and the 

 pharynx. 



The third — fifth visceral — arch (A. V) varies considerably in size. 

 Occasionally it is absent, or it may be quite well formed and compar- 

 able in diameter with the common carotid and pulmonary arches, but 

 normally, however, it is much smaller. It leaves the truncus arteri- 

 osus somewhat postero-ventrally relative to the systemic arch, and, 

 running parallel with the latter, ultimately fuses with it at the level 

 of the dorsal side of the pharynx, just proximal to the entry of the 

 ductus Botalli. 



The pulmonary — sixth visceral — arch (A. VI) in the Salamander 

 supplies the walls of the pharynx as well as the lungs. It does not run 

 quite parallel with the other arches, but after leaving the truncus 

 arteriosus it turns at first slightly posteriorly, then bends anteriorly 

 to the anterior border of the M. dilatator laryngis, at which point the 

 ductus Botalli is given off. The latter passes mesially to the muscle 

 and enters the systemic arch just distal to the point where the 'third* 

 arch enters. The ductus Botalli is perforated by a functional lumen, 

 and in fact one or two small cutaneous branches are given off from 

 it laterally, as well as the lateral pharyngeal artery from its mesial 

 side. The pulmonary arch then turns sharply posteriorly again 

 towards the lung. 



4. Branches of the Carotid Arch (Common Carotid) (PI. VI, figs. 

 37, 38, and PI. XIV). 



(Note. — The terms ''Internal'' and ^External' are applied to the two divisions 

 of the Common Carotid for sake of convenience^ although their distribution is much 

 more restricted than is the case in the Frog.) 



The common carotid emerges from the truncus arteriosus some- 

 what antero-dorsal to the systemic arch and passes laterally for about 

 4 or 5 mm. to the carotid gland, where it divides into its external and 

 internal branches. 



A. The external carotid (a.car.e.) artery leaves the gland on its 

 mesial side anterior to the entry of the common carotid, and runs 

 back parallel with the latter for a millimetre or so before turning 

 anteriorly. From this section of the vessel arises: 



(i) Ramus muscularis for the MM. rectus cervicis and interhyoideus 

 posterior. This branch divides almost immediately, so that the 

 branch for the former muscle passes mesially, dorsal to the thyroid 

 vein, while the other enters directly into its muscle, which covers this 

 neighbourhood ventrally. 



