306 



IlEPTILIA. 



column, receiving at the point where it begins 

 to take this direction from before backwards 

 the left branch of the right aorta, which forms 

 a loop in front of it. From the convexity of 

 this loop arises the left carotid. The two 

 other branches of the right aorta wind back- 

 wards, and join together in a similar manner 

 upon the right side of the neck, forming two 

 loops placed one in front of the other. The 

 right carotid arises from the convexity of the 

 loop. The subclavians are given off from each 

 aorta a little before their union, except in 

 crocodiles and the iguana, where they are 

 both derived from the right aorta. The 

 common trunk formed by the union of the 

 two aortas, which takes place just beyond the 

 apex of the heart, gives off in succession 

 numerous pairs of intercostal arteries. It 

 sends, moreover, shortly after its commence- 

 ment, an artery to the oesophagus, and 

 subsequently a small branch to the liver; 

 further backwards it gives off an artery 

 which soon divides into two branches : of 

 these the anterior supplies the stomach, the 

 spleen, the pancreas, and the duodenum ; 

 the posterior, which represents the anterior 

 mesenteric, is appropriated to the intestinal 

 canal. The aortic trunk then gives off in 

 succession the lumbar, the spermatic, the pos- 

 terior-mesenteric, which supplies the rectum, 

 and the renal, which are given off thus late 

 because the kidneys are situated very far back 

 in the abdominal cavity : lastly, it gives origin 

 to the iliacs and the middle sacral arteries. 

 The last-mentioned vessel may fairly be re- 

 garded as a continuation of the aortic trunk, 

 1'rom which the iliacs seem to be mere 

 branches ; a circumstance which is owing to 

 the excessive proportions of the tail when 

 compared with the extremities. 



In the Ophidian reptiles, the absence of 

 limbs, the existence commonly of a single 

 lung, and the extremely slender and elongated 

 form of the body, concur to render the distri- 

 bution of the arterial trunks very simple 

 throughout this order. These trunks, as in 

 the Chelonians and Saurians, are three in 

 number. Their first divisions, instead of being 

 double and symmetrical, are reduced to single 

 trunks. This is the case, for example, with 

 the pulmonary artery in those serpents that 

 possess but one lung, and also with the com- 

 mon carotid, and the vertebral in the entire 

 order. 



It is from the convexity of the right aorta, 

 and very near its origin, that the above 

 branches, destined to supply blood to the 

 head and neck, are derived. 



The right aorta then winds backwards, 

 passes above the oesophagus, and then running 

 obliquely inwards and backwards, it joins the 

 left aorta at a little distance beyond the apex 

 of the heart. 



The right aorta gives off, a little after its ori- 

 gin, a small artery that supplies a small round 

 glandular-looking mass situated in front of 

 the base of the heart, and subsequently to 

 another similar body of elongated form, situ- 

 ated beneath the jugular vein. It then gives 



off the common carotid, which is single in all 

 the Ophidia. A third artery is given off a 

 little further on, which is the common trunk 

 of the vertebral and anterior intercostals. No 

 other important artery is given off by the 

 right aorta, and when it joins the left aorta 

 its diameter is very small, so that the greater 

 portion of the blood that this vessel receives 

 from the heart is supplied to the organs 

 which are situated in front of that viscus : it 

 might therefore be properly named the carotid 

 artery. 



The carotid artery runs obliquely towards 

 the left side, and advances forward, closely 

 connected to the left jugular vein, between the 

 trachea and the oesophagus, and at length is 

 situated beneath the latter. It gives off a 

 great number of small branches to these parts, 

 and near the head divides into several small 

 arteries, which represent both the external 

 and internal carotids. 



When the right aorta approaches the verte- 

 bral column, it gives off, as stated above, a 

 considerable branch, which supplies the place 

 both of the vertebral arteries and the anterior 

 common intercostals. This artery advances 

 beneath the vertebral column, giving off 

 branches on both sides, opposite each inter- 

 costal space, both to the muscles and to the 

 vertebrae of the region which it traverses, and 

 only enters the vertebral column close to the 

 head. This vertebral and intercostal artery 

 likewise gives off recurrent branches, which 

 furnish intercostal vessels behind its point of 

 origin. 



The left aorta runs upwards, backwards, 

 and to the left side : passes beneath the oeso- 

 phagus, and afterwards beneath the lung until 

 it reaches beyond the apex of the heart, 

 where it receives the right aorta, and con- 

 tinues its course backwards. It continually 

 gives off branches corresponding to the inter- 

 costals and the visceral arteries : those which 

 furnish the stomach, the liver, and the pul- 

 monary sac or sacs, are given off successively 

 from the aorta in its course backwards, so 

 that there is nothing like caeliac axis. Nearly 

 opposite the pylorus it gives off the anterior 

 mesenteric, which runs parallel to the intestine 

 for half its length, to which it constantly 

 furnishes branches. Further backwards the 

 intestinal canal receives in succession three 

 other small branches from the aorta, which 

 gives off as it runs backwards arterial branches 

 to the kidneys, ovaries, and other viscera. 

 Arrived at the termination of the abdomen, 

 it passes on beneath the vertebra? of the tail, 

 in which it becomes gradually expended. 



Organs of Respiration. In several species 

 of Lizards the cavity of the fauces is much 

 enlarged by an expansion of the skin in front 

 of the larynx (fig. 216, rf.). These laryngeal 

 sacs, as they are called, appear to be recep- 

 tacles for air rather than food ; for, although 

 not connected with the larynx, they are ex- 

 traordinarily distended in rage, etc. 



Before the termination of the trachea, both 

 in the Coluber natr'uc and thiringicits, there is 

 a small blind depression, which, as was first re- 



