GENERAL CHARACTERS OF REPTILES. 55 



skeleton is imperfectly developed ; so that here we have an approach to the 

 character of the Invertebrata. 



27. The swimming-bladder, as it is commonly termed, of the Fish, is not 

 an organ sui generis ; but is ascertained, by comparison with the pulmonary 

 sacs of the lower Reptiles, to be a rudimentary lung. It does not, however, 

 give any assistance in the aeration of the blood, except in a few instances; 

 but seems to be in general subservient to the elevation and depression of the 

 body in its element. The heart of the Fish is extremely simple in its con- 

 struction, containing two cavities only ; and the course of the circulation is 

 equally simple. The blood which returns from the body in a venous condi- 

 tion, is received into the single auricle or recipient cavity ; and from this it 

 passes into the ventricle or propellent cavity. The latter forces it into a large 

 trunk, which subdivides into branches that are distributed to the branchial 

 arches on each side ; and in these it undergoes aeration. { Being collected 

 from the gills by returning vessels, the blood, now become arterial in its cha- 

 racter, is transmitted to the large systemic trunk, the aorta, by which it is dis- 

 tributed through the system, returning again to the heart, when it has passed 

 through the organs and tissues of the body. Hence it is evident that the 

 whole of the blood passes through the gills, before it goes a second time to the 

 system ; by which the imperfection of the aerating process itself is in some 

 degree compensated. There is a special provision, too, for renewing by 

 muscular power the stratum of water in contact with the gills ; continual cur- 

 rents being sent over them from the pharynx, with which their cavity com- 

 municates. It is worth noticing, that whilst, in the Osseous Fishes, there is 

 a single large external-gill opening on either side, with a valve-like opercu- 

 lum or gill-cover, there are, in the Cartilaginous Fishes, several slits on each 

 side of the neck, one corresponding with each branchial arch. Similar aper- 

 tures in the neck maybe seen in the embryo of Man and of other Mammalia, 

 as well as of Birds and Reptiles, at the time that the circulation is in the con- 

 dition of that of the Fish, the heart possessing only two cavities, and the 

 blood being first propelled through a series of branchial arches. 



8. General characters of Reptiles. 



28. The class of REPTILES is oviparous and cold-blooded, like that of 

 Fishes ; but the animals belonging to it are formed to breathe air, and to 

 inhabit the surface of the earth, the few which are adapted to make the 

 water their dwelling, being obliged to come to the surface to breathe. Al- 

 though they breathe air, however, their respiration is not usually so energetic 

 as that of Fishes, and their general activity is much less. The mechanism 

 for the inflation of their lungs is very imperfect. Being destitute of a dia- 

 phragm, they are obliged to force air into the chest, by a process resembling 

 deglutition or swallowing; so that, strange as it may seem, a Reptile may be 

 suffocated by holding its mouth open. The heart possesses three cavities, 

 one of which receives the blood from the lungs, and another from the general 

 system ; the arterial and the venous blood, contained in these two auricles 

 respectively, are transmitted to the third or propelling cavity, the ventricle, 

 where they are mixed ; and the half-arterialised fluid is then transmitted to 

 the system at large, a part being sent to the lungs. Thus only a portion of 

 the blood expelled from the heart is exposed to the influence of the air ; and 

 that which is transmitted to the body is very imperfectly arterialised. In 

 some of the higher Reptiles, as the Crocodile, the ventricle is double, as in 

 the superior Vertebrata; and the course of the circulation is so arranged, that 

 pure arterial blood shall go to the head, where it is most required, whilst a 

 mixed fluid is sent to the rest of the body. This plan exactly corresponds 



