VOL. XXVII.] PHILOSOPHICAL TRANSACTIONS. tJOl 



or points of support, towards the inferior circumference of the heart : the other 

 muscular fibres, which compose the heart, are in the manner of several columns, 

 as those of the human heart; they are situated internally in both sides, lying 

 obliquely from the right, where their tendons are about the arteries, to the left; 

 which demonstrates that their action is from the left to the right side, where 

 the orifices of the arteries lie open, to let the blood pass out. 



It has been said before, that the two auricles of the heart of the land tortoise 

 of America, make externally but one continued body; but that it has internally 

 two cavities, separated from each other by a muscular septum. This septum 

 separates them so exactly, that there is not the least communication between 

 them; so that the blood of either auricle does not mix with that of the other, 

 except in the ventricle of the heart. The right auricle is as large again as the 

 left; all the blood of the animal (that of the lungs excepted) passing through 

 it, to go into the heart; the left auricle receiving only the blood which comes 

 from the lungs, the pulmonary veins being very small. The internal part of the 

 auricles are furnished with small muscular columns, but particularly at their ex- 

 tremities, and situated in such a manner, that it is plain their action tends to 

 push the blood against the septum, where the conduits, which convey it into 

 the heart, are situated. 



There is in the bottom of the right auricle an oblong orifice, by which the 

 blood comes into its cavity from the great reservoir of the veins, situated on the 

 back part of the heart. This orifice is furnished with two semilunar oblong 

 valves, disposed in such manner, that when the auricle is relaxed, the blood 

 enters its cavity, but when contracted, they shut close, to hinder the blood 

 from returning into the veins : the orifice of the funnel, or the conduit into 

 the heart, is to be seen against the septum. The left auricle has exactly the 

 same structure as the right. It is in the bottom of this auricle, that the orifice, 

 common to the two pulmonary veins, is to be observed, furnished with two 

 semilunar valves; and against the septum to the right, that the funnel or con- 

 duit into the heart is situated, joining with the funnel of the right auricle. 

 These two conduits are separated from each other by the continuation of the 

 septum, which divides the auricles to the very ventricle of the heart, and is as 

 a support to the middle of the double valve, which covers their orifices in the 

 heart. 



It has been said above, that from the basis of the heart of the land tortoise 

 of America, proceed 4 great arteries. Of these, the first which presents itself, 

 (the tortoise being turned on his back) is the pulmonary artery : it is more on 

 the left side than the others, and is much larger for the space of an inch; then 

 it divides itself into two branches, of which the most apparent comes from the 

 VOL. V. 4 H 



