2io CIRCULATORY SECT. XXIII. 5. 



by fear, or even by voluntary eructation, a great quantity of air 

 is generated, by the fermentation, which inftantly fucceeds the 

 flop of digeftion. By the experiments of Dr. Hales it appears, 

 that an apple during fermentation gave up above fix hundred 

 times its bulk of air ; and the materials in the . ftomach are fuch 

 and in fuch fituation, as immediately to run into fermentation, 

 when digeftion is impeded. 



As the blood paiTes through the fmall veflels of the lungs, 

 which connect the pulmonary artery and vein, it undergoes a 

 change of colour from a dark to a light red ; which may be term- 

 ed a chemical change, as it is known to be effeiled by an ad- 

 mixture of oxygene, or vital air ; which according to a difcov- 

 ery ofDr Prieftley, pafTes through the mg.i(l membranes, which 

 conftitute the fides of thefe veflels. As the blood pafles through 

 the capillary veflels, and glands, which connect the aorta and 

 its various branches with their correfpondent veins in the ex- 

 tremities of the body, it again lofes the bright red colour, and 

 undergoes fome new combinations in the glands or capillaries, 

 in which the matter of heat is given out from the fecreted fluids. 

 This procefs therefore, as well as the procefs of refpiration, has 

 fome analogy to combuftion, as the vital air or oxygene feems 

 to become united to fome inflammable bafe, and the matter 

 of heat efcapes from the new acid, which is thus produced. 



V. After the blood has pafled thefe glands and capillaries, 

 and parted with whatever they chofe to take from it, the re- 

 mainder is received by the veins which are a fet of blood-abforb- 

 ing veflels in general correfponding with the ramifications of the 

 arterial fyftem. At the extremity of the fine convolutions of the 

 glands the arterial force ceafes ; this in refpedl to the capillary 

 veflels, which unite the extremities of the arteries with the com- 

 mencement of the veins, is evident to the eye, on viewing the 

 tail of a tadpole by means of a folar, or even by a common mi- 

 crofcope, for globules of blood are feen to endeavour to pafs, and 

 to return again and again, before they become abforbed by the 

 mouths of the veins-, which returning of thefe globules evinces, 

 that the arterial force behind them has ceafed. The veins are 

 furnifhed with valves like the lymphatic abforbents ; and the 

 great trunks of the veins, and of the lacleals and lymphatics, 

 join together before the ingrefs of their fluids into the left cham- 

 ber of the heart , both which evince, that the blood in the veins, 

 and the lymph and chyle in the lacleals and lymphatics, are car- 

 ried on by a fimilar force ; otherwife the dream, which was pro- 

 pelled with a lefs power, could not enter the veflels, which con- 

 tained the ftream propelled with a greater power. From whence 

 it appears, that the veins are a fyftem of veflels abforbing blood, 



as 



