SECT. XXIII. 5. OF THE CIRCULATORY SYSTEM. 309 



a light red : which may be termed a chemical 

 change, as it is known to be effected by an admix- 

 ture of oxygene, or vital air ; which, according to 

 a difcoyery of Dr. Prieftley, pafles through the moid 

 membranes, which conftitute the fides of thefe vef- 

 fels. As the blocd pafles through the capillary vef- 

 fels, and glands, which conned the aorta and its va^ 

 rious branches with their correfpondent veins in the 

 extremities of the body, it again lofes the bright 

 red colour, and undergoes fome new combinations 

 jn the glands or capillaries, in which the matter of 

 heat is given out from the fecreted fluids. This pro- 

 cefs therefore, as well as the procefs of refpiration, 

 has fome analogy to combuflion, as the vital air or 

 oxygene feems to become united to fome inflamma- 

 ble 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 remainder is receive \ by the veins, 

 which are a fet of blood-abforbing veflels in general 

 correfponding with the ramifications of the arterial 

 fyflem. At the extremity of the fine convolutions 

 of the glands the arterial force ceafes ; this in ref- 

 peft to the capillary veflels, which unite the extre- 

 mities of the arteries with the commencement 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 

 microfcope, for globules of blood are feen to endea- 

 vour 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 la&eals 

 and lymphatics, join together before the ingrefs of 

 their fluids into the left chamber of the heart ; both 



which 



