180 



In parting with its carbon which, by uniting with oxygen, forms the 

 carbonic arid that is thrown out during expiration, the blood loses its 

 dark and nearly purple colour, and becomes of a florid red, and its con- 

 sistence increases, from the escape of its hydrogen and of its aqueous 

 parts. Besides, as it absorbs a certain quantity of oxygen, it becomes 

 spumous and light : its concrescibility and plasticity increase, and on 

 coagulating, there is separated from it, a smaller quantity of serum. 



After parting with its hydrogen and carbon, ^hd combining with oxy- 

 gen and caloric, in its passage through the lungs, the blood, which has be- 

 come arterial, parts with these two principles, in proportion as in reced- 

 ing from the heart, it forms new combinations, and is converted into oxy- 

 des of hydrogen and carbon, which on receiving an additionl quantity of 

 oxygen, are changed into water and carbonic acid, when on being carried 

 along with the venous blood, into the pulmonary tissue, they are exposed 

 to the influence of the atmospherical air. 



The arterial blood becomes venous, by yielding its oxygen, when any 

 cause whatever suspends or slackens its course, as is proved by the fol- 

 lowing experiment of John Hunter. He tied the carotid artery of a dog, 

 with ligatures placed at the distance of about four inches from each other: 

 the blood contained in the portion of artery included between the two li- 

 gatures, on laying open this part of the vessel, at the end of a few hours, 

 was found coagulated and as dark as that in the veins. The blood con- 

 tained in an anetorismal sac, and which is frequently found in a fluid state, 

 when the internal coats of the artery are but lately ruptured, becomes 

 venous after remaining in it some time. The changes, however, which 

 the blood undergoes in its course through the arterial system, are not 

 very remarkable, owing to the rapidity with which it flows along those 

 vessels; there is less difference between the blood contained in an artery 

 near the heart, and that contained in an artery at a distance from that or- 

 gan, than in the blood taken from the veins near their extremities, and 

 from the great trunks which deposit it into the right auricle. The blood 

 in the small veins resembles arterial blood, and frequently in a very co- 

 pious bleeding, the colour of the blood, which, at first, is very dark, 

 gradually becomes less dark, till towards the end of the bleeding, it shows 

 nearly the same qualities as if arterial ; a phenomenon which, as is well 

 observed by the English writer already quoted, depends on the more easy 

 and rapid flow of the blood'of the arteries into the veins, in consequence 

 of the evacuation of the venous system. This observation is a complete 

 icfutation of the assertion of Bellini, who maintains, that when a vein is 

 wounded, the blood which comes from it, forms a double current which 

 flows out at the wound. The above opinion is maintained by highly dis- 

 tinguished phosiologists, as Haller and Spallanzani, who support it by 



* Sir Humphrey Davy gives the following results of experiments made on his own 

 respiration : 



After one strong expiration, his lungs contained 330 cubic inches ; after a natural 

 expiration, 970 cubic inches; after a natural inspiration, 11Q6 cubic inches; after a 

 strong inspiration, 3206. By a strong expira^on, after a deep inspiration, 1556 cubic 

 inches were thrown out of the lungs. After a natural inspiration, 643 cubic inches, 

 and after a natural expiration, 353. Mr. Thompson supposes that the quantity of air 

 usually contained in the lungs is 2294 cubic inches, and that 327 cubic inches enter or 

 escape in inspiration or expiration. If we allow 20 inspirations a minute, we should 

 have entering and escaping from the lungs in this time 6500 cubic inches, and in 

 twenty-four hours, 75,556 cubic inches, or 48 pounds. Godman, 



