of Animals of the genus Rana, 161 



demonstrated clearly that the heart has two auricles, divided by 

 a transparent membranous septum, possessing fibres that appear 

 to be muscular ; that these auricles communicate with the ven- 

 tricle by a common and very short passage, provided with three 

 semilunar valves ; and that they have no possible communication 

 with each other, excepting through the passage above the valves 

 common to both of them. 



The same fact as to structure may also be demonstrated, by 

 blowing air through either of the two pulmonary veins, which 

 return the blood from the lungs to the heart. The pulmonic 

 auricle, the smallest of the two, is thus distended, and not the 

 systemic ; or, by blowing air into the large sinuses into which 

 the venae cavae terminate, when the reverse of the preceding ex- 

 periment takes place ; and this, at the same time, shews that the 

 margin of the septum acts as a valve, and must prevent the 

 blood of one auricle passing into the other. 



But, even did not the margin of the septum perform the func- 

 tion of a valve, the blood from one auricle could not pass into 

 the other, the contraction of the two being synchronous; the 

 auricles first contracting, next the body pf the ventricle, and, 

 lastly, that part of the ventricle of a conical shape, which may 

 be considered almost as a second ventricle *. 



I have observed the same kind of structure of heart in the 

 bull-frog and the common frog. Whether it exists in all the 

 other species of the genus, I have not ascertained, but most 

 probably it does ; and, reasoning from analogy, the probability 

 is very strong that all the other genera of the order * Batraciens' 

 have a similar conformation, both of this vital organ and of the 



• I am almost induced to consider this part as a second ventricle, from its 

 peculiarities, which I am not aware have hitherto been noticed. It is. sepa- 

 rated from the body of the ventricle by three valves, of a semilunar form. 

 To the side of its cavity is attached a fleshy projection, or moveable septum, 

 above which it gives origin to four arterial trunks, viz. two aortae and two 

 pulmonary arteries, the former considerably larger than the latter, each pro- 

 vided with its own semilunar valve ; and the action of this part seems to me 

 to be as peculiar as its structure. When I have watched it, it did not appear 

 to contract simultaneously, but first one-half and then the other ; as if in- 

 tended, in conjunction with the various anastomoses of the arterial system, to 

 preserve a constant, though small, current of blood, to supply all the parts of 

 the body according to their various demands. 



APRIL — JUNE 1828. L 



