ARTERIES OF REPTILES. 517 



channels converge to form the 'branchial veins,' ib. b. which 



O ' 



re-enter the neck,, course along the dorsal walls of the pharynx, 

 and unite to form the ri;ht and left aortas, or roots of the common 



o 



median arterial trunk, ib. A. From the first or foremost branchial 

 vein is given off the carotid or cephalic artery, a*. The fourth 

 pair of vascular arches pass outward and backward, and each 

 divides, the larger portion, P, going to the lungs, the smaller 

 division pursuing its course to the back of the oesophagus, where it 

 unites with the third ( branchial vein,' and adds a small proportion 

 of unrespired blood to the contents of the aorta. 



In the Proteus the bulbus arteriosus divides into a pair of 

 vessels, each of which, as it diverges, again divides ; the anterior 

 division supplies the anterior gill : the posterior division bifurcates, 

 to supply the second and third gills. But, before distributing 

 the branchial capillaries, each artery communicates by a direct 

 anastomosing channel with the branchial vein ; in other words, 

 only a portion of each primary vascular arch is appropriated to 

 the gills, and a certain proportion of the blood goes from the 

 heart to the aortic trunk without being submitted to the respira- 

 tory process. The small artery to the slender simple pulmonary 

 sac is sent off from the hindmost branchial vein. The cephalic 

 arteries arise from the foremost branchial veins, and consequently 

 supply the brain with purer arterial blood than that which goes 

 to the body. 



The changes in the vascular arches, consequent on absorption 

 of the gills, are chiefly due to the enlargement of anastomosing 

 channels, between the inferent and efferent branchial trunks, like 

 those in Proteus. The course of these changes reduces the 

 arterial system in Menopoma to the condition of which Hunter 

 left the illustrations published in xx. vol. ii. The bulbus is 

 shortened, and the origins of the primary vascular arches 

 approximated : those of the third and fourth are blended together. 

 The foremost arch is smaller than the second or third : it sends off 

 an artery to the intermandibular space, a second to the side of the 

 head, a third to the pharynx, beyond which it bends abruptly 

 back, to enter the aortal root. The second and third vascular 

 arches are of equal size, wind round the wide pharynx, anterior 

 to the branchial opening, and unite on reaching its back part to 

 form the aortal root : this sends forward a small artery to the side 

 of the mouth, and, a little further on, the main carotid artery, 

 beyond which the roots converge backward, and unite to form 

 the aortic trunk. The hindmost vascular arch is the smallest, 

 and courses round the oesophagus, below or behind the branchial 



