ON ANIMAL LIFE. 739 



the blood, through the right auricle and ventricle of the heart, and enters the 

 lungs, to be tliei e more intimately mixed with it, and perhaps to be rendered 

 animal and vital ; while the blood receives from the air, in the same place, a 

 supply of oxygen, with a small portion of nitrogen, and emits some superflu- 

 ous carbonic matter, in the form of carbonic acid. The blood, thus rendered 

 arterial, returuing to the left side of the heart, is distributed thence to every 

 part of the system, supplying nutriment throughout, while the glands and ar- 

 teries secrete from it such tluids,as are become redundant, and such as are required 

 for particular purposes subservient to the animal functions. It is probably in 

 these processes that heat is evolved; for by experiments on living animals, it has 

 been found, that the blood, returning from the lungs, is not warmer than be- 

 fore its entrance into them : we must therefore suppose, that when the tlorid 

 arterial blood is, by some unknown means, converted, in the extreme ramifica- 

 tions of the arteries, into the purple venous blood, to return to the heart by 

 the converging branches of the veins, there is a much more considerable ex- 

 trication of heat, than in the conversion of venous into arterial blood, by the 

 absorption of oxygen and nitrogen in the lungs. If the chyle is actually con- 

 verted into blood in the lungs, it is here that we must look for the formation 

 of the red globules, those singular corpuscles, to which the blood owes its 

 colour, as it docs its power of coagulation to a glutinous lymph, mixed with 

 a less coagulable scrum. The red particles in the human blood are about 

 •a-oW of an inch in diameter, somewhat. oblong, and flattened; they have 

 usually the appearance of a dark point in the centre; but there is tome 

 reason to suspect that this is merely an optical deception. In a few animals 

 they are a little smaller, but in most of the amphibia, much larger and flatter 

 than in man. While the lymph remains fluid, after the blood has been with- 

 drawn from the vessels, these globules tend to subside, and to leave it semi- 

 transparent: hence arises the appearance of a buff coat on blood left to co- 

 agulate, whicli is thinner or thicker, accordingly as the globules are sooner or 

 later arrested in their descent. 



Themusclesareprobably furnished by the blood with a store of that unknown 

 principle, by which they are rendered ca])able of contracting, for producing 

 locomotion, or for other purposes, in obedience to the influence transmitted by 

 the nerves from the sensorium; the brain and nervous system in general arc 

 also sustained, by means of the vascular circulation, in a fit state for trans- 

 mitting the impressions, made by external objects on the senses, to the im- 



