316 



CIRCULATION OF NUTRITIVE FLUID. 





agency of a central impelling organ, as in other Fishes ; but by a power 

 which is scattered or diffused through various parts of the system of 

 blood-vessels, as in the lower Invertebrata. The respiratory apparatus, 

 also, is formed upon a type much lower than that of Fishes ; for it con- 

 sists simply of a dilatation of the first part of the alimentary canal, or 

 pharynx, upon the walls of which the blood is distributed in divided 

 streams, its cavity being filled with water, which serves to aerate the 

 blood. This is precisely the type, on which the respiration is effected, 

 in those lowest Molluscs, of which mention has just been made, as ex- 

 hibiting alternations in the direction of the circulating current ( 557). 

 In other respects, however, the arrangement of the vascular system in 

 this extraordinary animal corresponds with that which obtains in 

 Fishes. 



561. It is requisite that, in the class of Fishes, the whole of the 

 venous blood returned from the system should pass through the respi- 

 ratory organs before being again transmitted to the body ; since the 

 aerating action of the small quantity of air diffused through the water, 

 would otherwise be insufficient for its renovation. But in Reptiles, all 



of which breathe air during their adult condition, 

 the case is very different ; for if the whole current of 

 their blood were exposed to the atmosphere, before 

 being again sent to the body, the quantity of oxygen 

 conveyed into the tissues would be too great, and 

 would have an over-stimulating effect. The plan of 

 the Circulation is, therefore, differently arranged in 

 Reptiles. We find the heart to consist of three 

 cavities ; two auricles and one ventricle. From the 

 ventricle issues a single trunk, which speedily sub- 

 divides ; some of its branches proceeding to the lungs, 

 and others to the body. The blood which is trans- 

 mitted through this trunk, is of a mixed character, 

 as we shall presently see ; being neither fully aerated, 

 nor yet highly carbonized. It contains sufficient 

 oxygen, to stimulate the nervous and muscular sys- 

 tems of these comparatively inert animals ; whilst it 

 also contains enough of carbonic acid, to require 

 ventricle," receiving the being exposed to the atmosphere through the medium 



aerated blood from b, the . L V m & 



pulmonary auricle, and ve- of the lungs. The blood which has passed through 



nous blood from c, the sys- ,-1 -11 i i i i i xl,u 



auricle; and propel- the systemic capillaries, and which has been thereby 

 rendered completely venous, is returned to one of 

 art to the sys- the auricles the systemic by the vena cava. On 

 the other hand, the blood which has passed through 

 the capillaries of the lungs, and which has been thereby rendered com- 

 pletely arterial, is returned through the pulmonary vein to the other 

 auricle, the pulmonary. Thus one of the auricles exclusively receives 

 aerated, and the other carbonated blood ; and as both pour their con- 

 tents into the common ventricle, the blood which that cavity contains 

 and propels is of a mixed character. 



562. Various modifications of this form of Circulating apparatus 

 exist in the different groups of reptiles. In the lowest among them, 



