REPTILIA. 



305 



branches to the oesophagus and neighbouring 

 muscles, until it reaches the head, to which it 

 is distributed without previously dividing 

 into two principal branches, representing the 

 two carotids of" mammalia. 



This difference doubtless depends upon the 

 small size of the encephalon in these reptiles, 

 and it may be remarked that the comparative 

 smallness of the internal carotid, which is 

 here only a subordinate branch of the ex- 

 ternal carotid, is not compensated by the 

 size of the vertebral artery, which in the 

 Chelonians does not exist. This last cir- 

 cumstance will not be surprising when it is 

 remembered, that in birds the vertebral artery 

 only supplies the muscles of the neck, the 

 cervical vertebrae, and the spinal chord ; fur- 

 nishing in the cranium only a single small 

 branch, which is entirely expended upon the 

 medulla oblongata. 



As in birds, the internal carotid of the 

 Chelonians supplies all the parts of the en- 

 cephalon. 



There is a posterior communicating branch, 

 Avhich forms, in conjunction with its fellow, 

 a basilar artery ; which, as it is prolonged 

 backwards beneath the spinal chord, forms an 

 inferior median spinal artery. This last 

 azygos artery gives off near its origin a re- 

 current branch, which is the superior lateral 

 spinal. 



The external carotid, which is the principal 

 carotid trunk, resembles in its distribution 

 very closely the external carotid of mammalia ; 

 it may, however, be remarked, that after 

 giving off the lingual branch this artery pene- 

 trates into the temporal fossa through the 

 external carotid canal, and that it there 

 divides into two principal branches, one an- 

 terior, the other posterior ; the former is 

 appropriated exclusively to the head, and 

 supplies the place both of the internal maxillary 

 and of the temporal of mammalia. 



The posterior branch of the carotid repre- 

 sents an occipital artery, of which the cervical 

 branch is very greatly developed, so that this 

 occipital seems to be transformed into a re- 

 current cervical artery, which runs backwards 

 over all the dorsal region of the neck, giving 

 branches to the muscles, to the vertebrae, and 

 to the medulla spinalis, and ultimately anas- 

 tomoses with the superior ascending cervical 

 artery, derived from the subclavian. The 

 quantity of blood which is thus furnished to 

 the spinal chord of the neck by the recurrent 

 and ascending cervical arteries is very re- 

 markable. 



Branches of the right posterior Aorta. The 

 two posterior aortas, in the first instance, run 

 upwards and outwards towards their respective 

 sides, and then turning backwards approach 

 each other so as ultimately to unite nearly 

 opposite the fifth dorsal vertebra by a com- 

 municating branch which the left aorta fur- 

 nishes to the right. Before, however, receiving 

 this artery, the right aorta gives off several 

 branches corresponding with the anterior 

 intercostals. 



After receiving the communicating artery, 



VOL. IV. 



the right aorta runs backwards beneath the 

 vertebral column as far as the pelvis, giving 

 off in its course the following branches ; 

 1st, five arteries on each side analogous to 

 the intercostals ; 2d, the spermatic ; 3d, 

 several renal branches to the kidneys on each 

 side ; 4th, several small lumbar branches ; 

 5th, a small artery analogous to the posterior 

 mesenteric, which is distributed to the cloaca; 

 ultimately, the right posterior aorta terminates 

 by four branches, namely, 1st, the left ex- 

 ternal iliac ; 2d, the left internal iliac ; and 3d, 

 the common iliac of the right side. Between 

 the two latter trunks arises the caudal artery 

 analogous to the middle sacral. In the green 

 turtle (Chelonia Mydas} the six last inter- 

 costals are given off immediately from the 

 right aorta : but in the Emys europca their 

 origin is very different, the five last inter- 

 costals being derived from a common anterior 

 intercostal, which arises from the ascending 

 branch of the subclavian. This vessel runs 

 along the spine from before backwards, as is 

 the case in many birds, and ultimately termi- 

 nates by uniting with a posterior common in- 

 tercostal : this latter is a branch derived from 

 the iliac artery. 



The divisions of the internal arteries re- 

 semble very closely what is found in mammalia. 

 These vessels first separate into two branches, 

 one of which gives off vessels to the bladder 

 and to the cloaca ; the other plunges into the 

 pelvis, and apparently represents the ischiadic 

 and posterior iliac arteries : the external iliac 

 runs forward along the edge of the pelvis, 

 giving off the analogue of the epigastric, from 

 which arises the anterior iliac. Leaving the 

 pelvis, the external iliac takes the name of 

 crural, and after giving off the circumflex 

 arteries, and the profunda femoris, continues 

 its course, in all respects comparable in the 

 remainder of its distribution to what is found 

 in mammalia and birds. 



The left {Visceral) Aorta. The left aorta 

 furnishes large arteries to the principal viscera 

 of the abdomen, to which it is almost entirely 

 distributed. As soon as it has passed the 

 cardia it divides into three branches ; of these 

 the first, which is the smallest, furnishes a 

 twig to the oesophagus, and is then distributed 

 to the stomach, representing the coronary 

 artery of mammalia. 



The second, which is almost as large as 

 the trunk from which it arises, supplies the 

 intestines, the spleen, the pancreas, and the 

 liver. 



The third branch, intermediate in size be- 

 tween the two others, is the communicating 

 artery, given off to join the right aorta, and 

 from which no branches are furnished. 



In the Saurian reptiles the distribution of 

 the arterial trunks differs but little from that 

 above described. 



In Lizards the two aortas advance forwards 

 out of the thorax ; that of the right side after 

 dividing into three branches, that of the left 

 without any such division. The left aorta 

 winds backward upon the side of the neck, 

 and afterwards runs along the vertebral 



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