CIRCULATION IN FQTUS 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 Reprizzs, 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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