REPTILES. 



each of thefc cladcs, which are in an exaft ratio to the 

 degree of their other qualities. 



The circulation i« effefted by means of mufcular powers, 

 whicli are applied particularly to the arterial fyllem ; the 

 venous feemiiig to be merely paffive. A hollow mufclc, 

 called a wnlricle, poilefling great and continued irritabihty, 

 and contracting forcibly on the blood whenever it arrives, 

 is placed at the union of the venous and the correfponding 

 arterial trunk. Valves are placed at its two openings ; one 

 valve allows the blood to enter, and prevents it from going 

 back again, while the other permits its exit, and cuts off 

 the retirrn. The ventricle then cannot contraft without dif- 

 tcnding the arteries, pufhing forwards the blood, which they 

 contain already, and thus producing the /)u^. The ven- 

 tricle, having expelled the blood which irritated it, is re- 

 laxed a-id dilated, and then immediately filled with a frclh 

 quantity of blood from the veins. Before entering the ven- 

 tricle, the vein is dilated into a mufcular fac, called the 

 eurk/e, with much lefs confiderable mufcular fides. This 

 is irritated by the blood received from the vein, contrafts on 

 it, and fends it into the ventricle. It is hardly neceffary to 

 add, that the auricle and ventricle contraft alternately. 



Animals with a fimple circulation have a fingle ven- 

 tricle ; but they poflefs fometimcs two auricles. When the 

 circulation is double, there may be a ventricle at the origin 

 of each artery, or at that of one only. Mammalia and birds 

 have two, and the fepias among the mollufca. 



All other animals have a ventricle at the origin of one 

 only of the two arteries, but not of the fame in all. It is 

 placed, in fiihes, at the origin of the pulmonary artery ; in 

 the mollufca at the origin of the artery of the body, or the 

 aorU, which is the name of tkat artery. 



The union of the auricle and ventricle conftitutes the 

 /leqri. We find, therefore, in fifhes and mollufca, a fimple 

 heart, pulmonary in the former, aorlic in the latter. Rep- 

 tiles, alfo, have a fimple heart, at once pulmonary and 

 aortic. Mammalia, birds, and fepiae, have a double heart, 

 or rather two, a pulmonary and an aortic, hearts. 



In mammalia and birds the two hearts are united, and 

 form one mafs, which commonly bears the name of heart, 

 as if it were a fingle organ. This is not the cafe in the 

 fepiae. See Vf.RMEs, m Comparative Anatomy. 



We may now underftaud what naturahits mean, when they 

 fay that mammalia and birds have a heai-t with two auricles 

 and two ventricles ; reptiles and fifhes a heart with a fingle 

 auricle and ventricle. The latter phrafe, befides comprifiiig, 

 in a common expreffion, two things really very different, 

 contains alfo an error of fad ; for many reptiles have two 

 auricles. 



RefpeiSing the hearts of mammaha, birds, fiflies, and mol- 

 lufca, fee the articles Mammalia, Birds, Fishes, and 

 Vermes, in Comparative Anatomy. For the phyfiology of 

 the circulation, the powers and aftions of the heart, and 

 the other organs concerned in it, fee Circulation and 

 Heart. 



The eflential ftruftare of the heart is the fame in all 

 animals ; there are only modifications of greater or lefs im- 

 portance. It is covered externally by a fraooth membrane, 

 the reflefted portion of the pericardium ; its cavities are 

 lined by another fmooth membrane, continuous with the 

 linings of the blood-velfels ; and there is more or lefs muf- 

 cular fubftance interpofed between them. The exiftence 

 of the pericardium is as general as that of the heart ; its 

 nature and difpofition are fo nearly alike in all animsls, that 

 it is not worth while to notice the modifications. 



Of the Heart. — In the different orders of reptiles, the heart 

 is iound to vary in fome parts of its itrufture ; there is, 



however, no eflential difieronco, when viewed in relat:( n to 

 its funcliotis. The three firll orders have a heart confilling 

 of two auricles and a fingle ventricle, which is fometimee 

 divided into many cavities, communicating with each other. 

 On the other hand, in the batracians there is but a Cngle 

 auricle and ventricle, with its cavity of a fimple form. We 

 lliall enumerate fucceflively the differences of llrudture in 

 the four orders of thisclafs of animals. 



Firfl in the chelonians. In the animals conr.pofing this 

 order, the heart has a form altogether peculiar. The length 

 of the organ is much exceeded by its breadth ; in fome in- 

 flancts, it relemblcs the fegment of a fphere ; in others, it 

 is of a fquare but elongated form, and curved in its longi- 

 tudinal direftion. In its natural fituation, it is fo ind be- 

 neath the lungs, in front of the liver, and partly between 

 the two lobes of the latter. The pericardium, which is 

 capacious and ftrong, is in contaft with the membrane iu- 

 veiling thofe organs, and is as firmly adherent to it as the 

 pericardium is to the diaphragm in man. 



The magnitude of the two auricles is much greater in 

 proportion, than in any animals of the clafles of mammalia 

 or birds ; the capacity of each is at leall equal to that ©f 

 the ventricle ; they are fituated in part above the latter, 

 projefting upon its lateral and anterior parts. They poflefs 

 fomewhat of a rounded form, are without any appendix, 

 and have their parietes thin, with fome flefhy fafciculi in 

 their flrufture. The right auricle, which exceeds in a 

 trifling degree the fize of the left, receives, by a fingle 

 opening at its upper part, the blood returning from the 

 body. Two valves are placed around the borders of the 

 opening, giving it tlie appearance of a fimple fifl'ure. The 

 pulmonary veins alone terminate in the oppofite auricle ; 

 their termination is provided in the fame manner with two 

 valves. A fimple membranous partition feparates the cavi- 

 ties of the two auricles and their openings into the ven- 

 tricle. It is on the ventricle that depends the form which 

 we have defcribed as belonging to the heart. Its cavity is 

 very fmall in comparifon to its fize, which is owing to the 

 great thicknefs of the parietes. Thefe are found to be 

 compofed exteriorly of a moderately thick layer of fibres, 

 which have a direftion parallel to the external furface of 

 the ventricle. Beneath thefc, there" are other numerous 

 mufcular fafciculi, varying in their direftion, but proceed- 

 ing principally from the fuperior to the inferior furface ; 

 the greater number of them are only contiguous, or feparated 

 from each other, allowing the blood to pafs through the inter- 

 vals formed between them as through a fponge. It refults 

 from this itrufture, that the cavity of the ventricle is dimi- 

 niflied to one-third of its volume ; it occupies the middle 

 and right fide of its bafe. In its greater part, it is lined 

 by a continuation of the membranous fold which covers 

 the auricular orifices, performing to them the ofSce of a 

 valve ; it is of a fquare form, attached at the middle of its 

 external furface to the partition between the auricles, and 

 by its fuperior and inferior fides to the correfponding pa- 

 rietes of the ventricle ; it is only loofe and unconnected in 

 its right and left borders. The firft is extended over the 

 opening of the auricle on the fame fide, and the latter 

 over that of the oppofite auricle ; fo that thefe openings 

 appear in the ventricle feparated by the breadth of the fold, 

 while in the auricles there is but the thin feptum inter- 

 vening between them. The blood returning from the lungs 

 into the left auricle is direfted into the ventricle by means of 

 the valve belonging to the former, in a courfe direftly con- 

 trary to that leading to the opening belonging to the arte- 

 ries of the body. It mull, therefore, necefiarily pafs through 

 the whole cavity of the ventricle, from the left to the right, 



and 



