KEPTILES. 



Tlie bladder always receives the urine by its neck, or by 

 a begiiuiiiig of an uretlira, which opens immediately into 

 the cloaca. 



There arc two large membranous bags in the frog and 

 toad, occupying the lituation of the urinary bladiier, gene- 

 rally confidercd as lucii, and fo defcribed by Blumenbach 

 and Cuvier. Townfon doubts whether they ought to be con- 

 iidcrcd as part of the urinary apparatus. (See his Traft*? 

 and Obfervations, p. 66. tab. 3.) They have no conncftion 

 with the ureters. Indeed it is very clear, that the latter 

 tubes open on the fuperior furface ; while thefe two recepta- 

 cles terminate on the inferior furface of that inteftiiie. They 

 contain a pure water. Their fi/.e, which exceeds all ordi- 

 nary proportion to the bulk of the kidney, renders it like- 

 wife probable that they are not receptacles of urine. 



Renal CapJ'ules. — The parts to which this name has 

 been given, and which are found in the three firll orders of 

 reptiles, arc Itil! fmaller in proportion, than in mammalia and 

 birds, and are completely feparate from the kidnies. In the 

 cheloniansthey arc connected to the emulgent veins. In the 

 faurians and ophidians they lie in the fold of the peritoneum, 

 which unites the ovaries and ovidufts. 



For a defcription of the fringed tatty appendices, which 

 are found in the batracian order near the teilides and ovaries, 

 and the vedels of which join the emulgent veins, fee the ac- 

 count of the omenta. 



Organs of Circulation. — The whole oi nutrition is effedted 

 at one operation in zoophytes : chyle pafies into the parts 

 in proportion as it is made; in infefts alfo it bathes 

 them as foon as it is formed, and they appropriate it. In 

 the fuperior animals, there is an intermediate operation : a 

 particular fluid only, always moving in a certain fyilem of 

 veffels, immediately nourifhes the parts ; and this fluid is 

 renewed by the chyle. The motion of this peculiar fluid, of 

 this blood, IS called circulation j — a pi-ocefs confined to the 

 fuperior clafles, that is, to the vertebral animals, the mol- 

 lulca, the worms, and the crullacea. 



There are two principal points for our confideratiofl in the 

 circulation ; its agents, and the routes of the blood. 



That-part of the latter is particularly interefting, which 

 conducts the blood to the refpiratory organ. One of 

 the chief purpofes of the circulation is to conduft the blood 

 conllantly, in greater or fmaller quantity, into an organ, 

 where it may undergo the mediate or immediate aftion of 

 o>f)'gen ; and, as the qualities of the blood depend much on 

 the degree of force of this aftion, and in the modification 

 which the blood receives from it, while all parts of the body, 

 being nourithed by this blood, partake of its qualities, it 

 follows that the whole nature of an anjmal will be in fom.e 

 fort determined by the diilribution of its circulating organs, 

 and by the route which this diltribution marks out for the 

 blood. 



Hence arifes the importance of the llrufture of the heart, 

 in reference to natural hiftory, and the correftnefs of the 

 characters drawn from it for the formation of clafles. Men 

 of genius had forefeen, rather than demonfl;rated this im.- 

 portance ; but it has been eftabliihed on rational principles 

 only m modern times. 



The circulations through the body and the lung are 

 called, refpeftively, the great and the minor. In the for- 

 mer, all the blood returning from the body by the veins, 

 which joining together from all parts, ultimately end in 

 one trunk, goes again to thefe parts by the arteries, of which 

 a common trunk is gradually divided and fub-divided, until 

 the lad divifions, as well as their union with the roots of 

 the veins, efcape the eye. 



If the common trunk of the veins communicated direftly 



with the common arterial trunk, there would be a fingle ar- 

 culation ; the blood brouglit back to the centre, would be 

 fent again immediately to the parts, to return again direftly, 

 and fo on ; but this never takes place entirely. - 



Before the blood; brought back to the common trunk of 

 the veins, can again enter that of the arteries, it muft be fent 

 in part, or altogether, to the pulmonary organ, in order to 

 undergo the aftion of the atmofphere. 



If the circulating organs be fo arranged that every drop 

 of the blood goes through the lung, by the minor circulation, 

 before it can enter the arterial trunk, the common trunk of 

 the veins of the body fending all its blood into the pulmo- 

 nary arterial trunk, whole ultimate ramifications commu- 

 nicate with veins united into a common trunk, fending all 

 its blood into that of the arteries of the body, there is a 

 ilouble circulation. 



If, on the contrary, the common trunk of the general 

 veins, inilead of bemg diftributed entirely to the lung, 

 lliould only fend to it a branch, while the reft of its blood 

 (hould go diredly into the conunon trunk of the general 

 arteries, the minor circulation would be only a fradtion of 

 the great, more or lefs confiderable, according to the fize 

 of the branch devoted to it. In each circuit of the blood, 

 refpiration would be exercifed on a part only of this fluid, 

 and the reft would go again into the body by the arteries, 

 without having palled through the lung. This blood, and 

 the parts nouriflied by it, would participate lefs, ceteris 

 paribus, in the qualities which refpiration could impart to it. 

 This is what takes place in reptiles ; their pulmonary cir- 

 culation is only a fraction of the great, more or lefs confi- 

 derable in the ditt'erent genera. The other clafles, -viz. the 

 mammalia, birds, fiflies, mollufca, and worms, have a double 

 circulation, and no part of their blood can return into the 

 great, until it has gone through the minor circulation. 



Yet we are not to conclude that the ultimate effeij of 

 refpiration is the fame, becaufe circulation is the fame. The 

 mode of refpiration may be dift"erent, and, as this is one of 

 the factors, the produft will be afFefted by its alteration. 

 All the animals lalt enumerated have an entire pulmonary 

 circulation, while, in reptiles, it is only a fraction : let them 

 be, for example, as i to ^. Now filhes, mollufca, and 

 worms, breathing in water, and that oxygen only, which 

 is mixed and contained in this water, may be confidered as 

 having a half-refpiration, while reptiles, breathing air itfelf, 

 have an entire one. An entire refpiration, multiphed by a 

 half-circulation, and a half-circulation by an entire refpira- 

 tion, give the fame produft ; which is, in both cafes, a half- 

 oxygenation of the blood, ufing this term merely to exprefs 

 the changes taking place from breathing. 



Mammalia have an entire circulation and refpiration ; and 

 confequently an entire oxygenation. The quantity of the 

 latter is even greater in birds m confequence of the peculiar 

 manner in which air is introduced into all parts of their body. 

 The fraft ion of -A is only adopted for the purpofe of il- 

 luftration ; the quantity probably varies in the different 

 genera of each clafs, and cannot be rigoroufly appreciated. 

 By thefe confideralions we may eilimate, and in a manner 

 calculate, the nature of each animal. As refpiration gives 

 to the blood its heat and energy, and through its medium 

 imparts excitability to the organs, its quantity will deter- 

 mine the degree of vigour in the animal funft ions. Hence 

 we deduce the great force of the moving powers, the acute- 

 nefs of the fenfes, the rapidity of digeftion, and the heat of 

 the paflions in birds. Hence the more moderate degree of 

 all thefe in the mammalia ; hence the inertnefs, the inactivity, 

 and apparent Itupidity, of the other clafles. Hence, too, 

 the various modifications of vital temperature natural to 

 4 Z 2 each 



