368 OXYGEN ATION SECT. XXXVIII. 2. i 



Dr. James JefFray, and by Dr. Forefter French, in their inaugu- 

 ral diiTertations at Edinburgh and at Cambridge ; who have de- 

 fended the contrary opinion in an able and ingenious manner ; 

 and from whofe Thefes I have extracted many of the following 

 remarks. 



Firit, by the late difcoveries of Dr. Priedley, M. Lavoifier, 

 and ether philofophers, it appears, thar the bafis of atmofpher- 

 ical air, called oxygene, is received by the blood through the 

 membranes of the lungs ; and that by this addition the colour 

 of the blood is changed from a dark to a light red. Secondly, 

 that water poflefles oxygene alfo as a part of its compofition, 

 and contains air like wife in its pores ; whence the blood of fifh 

 receives oxygene from the water, or from the air it contains, by 

 means of their gills, in the lame manner as the blood is oxygena- 

 ted in the lungs of air-breathing animals ; it changes its colour 

 at the fame time from a dark to a light red in the veiTds of their 

 gills, which conftitute a pulmonary organ adapted to the medi- 

 um in which they live. Thirdly, that the placenta confiils of 

 arteries carrying the blood to its extremities, and a vein bringing 

 it back, refembling exactly in ftrudlure the lungs and gills above 

 mentioned , and that the blood changes its colour from a dark 

 to a light red in paiTing through thefe veflels. 



This analogy between the lungs and gills of animals, and the 

 placenta of the fetus, extends through a great variety of other 

 circumftances ; thus air-breathing creatures and fiih can live 

 but a few minutes without air or water ; or when they are con- 

 fired in fuch air or water, as has been fpoiled by their own 

 relpiration ; the fame happens to the fetus, which, as foon as 

 the placenta is feparated from the uterus, mud either expandjts 

 lungs, and receive air, or die. Hence from the ftruciure, as 

 well as the ufe of the placenta, it appears to be a refpiratory or- 

 gan, like the gills of fiih, by which the blood in the fetus be- 

 comes oxygenated. 



From the terminations of the placental veflels not being ob- 

 ferved to bleed after being torn from the uterus, while thofe of 

 the uteras effufe a great quantity of florid arterial blood, the 

 terminations of the placental veflels would feem to be inft-rtecl 

 into the arterial ones of the mother ; and to receive oxygenation 

 from the pafling currents of her blood through their coats or 

 membranes ; which oxygenation is proved by the change of the 

 colour of the blood from dark to light red in its paflage from 

 the placental arteries to the placental vein. 



The curious ftruciure of the cavities or lacunae of the placen- 

 ta, demonftrated by Mr. J Hurrer, explains this circumftance. 

 This ingenious philofopher has (hewn, that there are numerous 



cavities 



