602 PHILOSOPHICAL TRANSACTIONS. [aNNO 17 10. 



right side of its trunk, and turns itself over towards tlie left side, accompanying 

 the aorta inferior sinistra, till it has pierced the pericardium ; after that, it unites 

 with the left branch of the trachea arteria, which it accompanies through all the 

 extent of the left lobe of the lungs. The other pulmonary branch, going out 

 of the left side of its trunk, turns itself over immediately across upon the other 

 arteries, from the right to the left, to join the aorta inferior dextra, till it has 

 pierced the pericardium, where it joins to the right branch of the trachea 

 arteria, which it accompanies through the whole extent of the right lobe of the 

 lungs. 



One thing seems very remarkable in this pulmonary artery; viz. that though 

 its trunk, in going out of the heart, be more than double the diameter of the 

 arteria aorta sinistra, yet the two branches which it sends to the lungs, have not 

 either of them one third of the diameter of the aorta sinistra. In the arteries, 

 which I have filled with wax, the trunk of the pulmonary artery is between ^ 

 and 8 lines diameter; the aorta sinistra 4 and a half; and the pulmonary 

 branches, after having pierced the pericardium, have only one line and a half 

 diameter : yet this artery does not produce any other branch ; all the blood, 

 which enters from the heart into its trunk, is carried into the two lobes of the 

 lungs, and no where else. The reason of such disproportion I cannot guess ; 

 but this is matter of fact, since it is the same in all : yet, if I may be permitted 

 to conjecture, it seems to me that it may be attributed to the alteration that 

 happens to the branches of the trachea arteria, when the tortoise stretches forth 

 his head out of the shell ; for these pulmonary branches making a half circle 

 before they join with the trachea arteria, when the animal's head is drawn in, 

 the extension which happens to the branches of the trachea arteria, when the 

 animal goes out of the shell, turns these half circles into sharp angles ; so that 

 thereby the passage of the blood is somewhat interrupted, and consequently the 

 blood, which passes continually from the heart into the trunk, not being capable 

 to return back, because of its valves, must out of necessity dilate this trunk 

 more than the other arteries. And what persuades me that there does not go 

 into the lungs of this animal, more blood than that quantity which the pulmon- 

 ary branches can admit by their small diameter, and not the quantity which the 

 diameter of their trunk could furnish, is, that the pulmonary veins, which bring 

 back all the blood of the lungs into the left auricle of the heart, have not either 

 of them quite two lines diameter, which is very proportionable to the size of 

 the two pulmonary branches of the arteries. 



The second artery proceeding from the basis of the heart, is that which I 

 call the left aorta : it ascends, as it comes out of the heart, together with the 

 left pulmonary, till they have pierced the pericardium; after which, it makes a 



