CIKCULATION IN FCETUS AND IN REPTILES. 243 



presently find that there is but a single, instead of a double, 

 heart ; and that the organ which is absent is sometimes the 

 systemic, and sometimes the respiratory heart. 



283. Previously to birth, when the lungs are not yet dis- 

 tended with air, and the aeration of the blood is provided-for 

 in other ways, the circulation takes place on a different plan 

 from that on which it is afterwards performed. There exists 

 at that period an opening in the partition between the two 

 auricles, by which they have a free communication; and 

 there is also a large trunk which passes from the right 

 ventricle into the aorta. By these channels, the blood which 

 is received from the systemic veins can pass at once into the 

 aorta, without going through the pulmonary vessels. But 

 when the young animal begins to breathe, these communi- 

 cations are speedily obliterated; the blood is transmitted 

 through the pulmonary vessels to the lungs ; and the whole 

 circulation takes place upon the plan just described. There 

 are occasional instances, however, in which the communica- 

 tion between the auricles remains open, so that the double 

 circulation is never perfectly established ; for a portion of the 

 blood is allowed to pass from the right to the left side of the 

 heart, without being aerated in the lungs, so that the blood 

 which is sent to the system contains a mixture of venous with 

 the proper arterial fluid, a state which will be presently seen 

 to be natural in the Reptile. Such cases are recognised by 

 the blueness of the skin, the lividity of the lips, and the 

 indisposition to bodily or mental exertion. Persons affected 

 with this malformation seldom reach adult age. 



284. In the class of REPTILES, there is not a complete 

 double circulation ; for a mixture of arterial and venous blood 

 is sent alike to the lungs and to the general system ; and no 

 part is supplied with the pure arterialized fluid. In general 

 the heart contains only three cavities, two auricles and one 

 ventricle (fig. 133). One of the auricles receives the venous 

 blood from the system ; whilst the other receives the arterial- 

 ized blood from the lungs. Both these pour their contents 

 into the same ventricle, where they are mingled together; 

 and this mingled blood is transmitted, by the contraction of 

 the ventricle, partly into the lungs, and partly into the aorta 

 (fig. 130). In some Reptiles there is a partial division of 

 the ventricle, so that the mixture of the arterial and venous 



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