COMPARATIVE ANATOMY. 



very close to the stomach in reptiles, as 

 it is found in the mesentery of the frog ; 

 neither is it very uniformly situated in 

 fishes. 



In the Crustacea the liver is large, and 

 consists of blind tubes, opening into the 

 commencement of the intestine. It forms 

 the soft high flavoured substance of the 

 crab and lobster. 



A liver exists in all the mollusca, and 

 is very large ; but this class has no gall- 

 bladder. The liver is supplied with 

 blood from the aorta, and there is conse- 

 quently no vena portarum. 



It is a completely mistaken notion, that 

 the black fluid of the cuttle-fish is its bile. 

 The ink-bag is indeed found between the 

 two lobes of the liver in the sepia octo- 

 pus ; and in front of them in the cahnar ; 

 but in the common cuttle-fish ("sepia 

 officinalis"), it is at a considerable dis- 

 tance from this organ. 



The real bile is poured, as usual, into 

 the alimentary canal. 



The structure of the pancreas in the 

 mammalia, in birds, and in reptiles, is 

 the same, on the whole, as in the human 

 subject ; its form and size, its colour and 

 consistence, and its division into lobules, 

 exhibit some slight and unimportant vari- 

 ations. 



The termination of its duct or ducts is 

 distinct in birds from that of the D. cho- 

 ledochus. In the mammalia they generally 

 open together, or there is a branch termi- 

 nating in the D. choledochus, and another 

 opening into the intestine, as in the dog 

 and elephant ; or they may be quite dis- 

 tinct, as in the hare, porcupine, and mar- 

 mot. They may be separate or distinct in 

 different individuals of the same species, 

 as in the monkeys. 



The skate and shark have a pancreas 

 similar to that of the three first classes of 

 red-blooded animals. In other fishes the 

 situation of this organ is occupied by nu- 

 merous small tubes, called the caecal ap- 

 pendices, or pyloric caeca ; which afford a 

 copious secretion, analogous, no doubt, to 

 the pancreatic liquor. The internal sur- 

 face of these tubes becomes very red on 

 injection, and possesses a glandular and 

 secreting appearance. Their number va- 

 ries from one to several hundreds. 



The description of the organs which 

 are concerned in assimilating the food, 

 and in converting it into chyle, will be 

 naturally followed by that of the blood- 

 vessels, which carry it to all parts of the 

 body; of the organs of respiration, which 



subject it to certain important changes ; 

 and of the absorbent system. 



ORGANS OF CIRCULATION-. 



A perfect circulating system, to which, 

 on the one hand, fluids are brought by 

 the absorbents to be converted into blood; 

 and from which, on the other side, vari- 

 ous juices are separated in glands, and 

 viscera of a glandular structure, appears 

 to. belong universally and exclusively to 

 red-blooded animals. A pericardium ex- 

 ists in all these animals. Parts of such 

 a system, particularly a heart, and cer- 

 tain vessels connected with it, are found 

 in some genera of the white blooded 

 classes. 



It has been supposed that the amphi- 

 bious animals of this class, and the ceta- 

 cea, have an open foramen ovale, like that 

 of the foetus, in their septum auricula- 

 rum. And the necessity of such an open- 

 ing has been inferred from their way of 

 life, since they often pass a considerable 

 time under water without breathing. This 

 supposition has been fully refuted by the 

 repeated dissection of adult animals of 

 this kind ; which has shewn that an ex- 

 ception from the general rule very rarely 

 occurs. 



In several genera and species of web- 

 footed mammalia, and cetacea (that is, 

 in the common and sea-otters, in the dol- 

 phin, &c.) particular vessels have been 

 observed to be considerably and con- 

 stantly enlarged and tortuous. This struc- 

 ture has been principally remarked in the 

 inferior vena cava ; where there can be 

 no doubt that it serves, while the animal 

 is under water, to receive a part of the 

 returning blood, and to retain it until 

 respiration can be again performed, and 

 the lesser circulation be thereby again 

 put in action. 



There are some remarkable circum- 

 stances in the distribution of particular 

 arteries in certain animals of this class. 

 We may notice, as the most singular of 

 these, the rete mirabile, formed by the 

 internal carotid at its entrance into the 

 cranium, in several ruminating biscula, 

 and carnivorous animals; and that divi- 

 sion of the arterial trunks of the extre- 

 mities, which has been observed by Mr. 

 Carlisle in the slow-moving animals, viz. 

 the sloths, and lemur tardigradus. The 

 arteries of the arm and thigh, in these 

 cases, divide, as they leave the trunk, 

 into numerous parallel branches, which 



