44 PHILOSOPHICAL TRANSACTIONS. [anNO 1703. 



to the naked eye, because I have hitherto found the naked eye unable to dis- 

 cover the extremities of the arteries and veins when the blood itself was mov- 

 ing in them, in the transparent parts of the omentum or mesentery of qua- 

 drupeds, or in the lungs of live frogs or lizzlrds ; or after death, when the blood 

 has been retained in their lungs in the following manner: on making an incision 

 into the bodies of these animals their lungs will start out, and be distended with 

 inspired air; on these as quick as you can pass a ligature, i. e. a waxed thread, 

 and tie it firmly towards the upper part of the lobe, as near the heart as pos- 

 sible ; when the lungs of frogs and lizards are dried, thus distended, you may 

 examine them with the microscope, and they will appear as represented, plate 2, 

 fig. 1st, 3d, 4th, 5th, and 6th. The 1st and 2d figures show the difference in 

 the magnitude of the extremities of the veins and arteries of the lungs, and 

 those of the foot of a frog viewed with the same microscope. 



Hence it appears that the communications between the arteries and veins of 

 the lungs are more open than those of other parts, at least in the feet of frogs ; 

 and till it can be shown that melted wax can be as easily injected from the arte- 

 ries to the veins of other parts in a human body and quadrupeds, I shall be in- 

 clined to think the communications between the pulmonic arteries and veins in 

 general are more open than the arteries and veins of other parts, excepting the 

 spleen and penis. This patent communication of the arteries with the veins of 

 the lungs, shows how those vessels transmit the blood in equal time and quan- 

 tity, with the blood that moves in the rest of the blood-vessels of the whole 

 body, in a healthful state. And hence it is, that when any of the blood-vessels 

 of the lungs are straitened or totally compressed, the remaining unobstructed 

 blood-vessels are forced to discharge more than they were wont, and in time 

 those vessels become sufficiently dilated to supply the defect. The like happens 

 in the communicant branches of the arteries of any part, when some consider- 

 able branch or trunk is tied up, as in the operation for curing the aneurism 

 mentioned in N'' 280. 



Thus we find that the structure of the parts of animal bodies, is not only suf- 

 ficient to perform the ordinary operations of nature, but their organs are so 

 wonderfully contrived, that notwithstanding considerable parts of those organs 

 are obstructed, yet their neighbouring parts, as the blood-vessels in the case 

 before us, become capable of supplying the defect. And this indeed exacts our 

 gratitude as well as admiration of the Divine Architect. 



Explanation of the Figures. — Fig. 1 represents that part of the 5th figure at 



D done by a larger magnifying glass ; a the arteries ; b the veins of a 



frog's lungs, prepared as above mentioned; c their inosculations with each 

 other ; d the area of the microscope, as it appears to the naked eye. 



