THE BLOOD. Ill 



Tliis experiment also illustrates another point, namely, 

 that while contact with foreign bodies causes the red cor- 

 puscles to part with their paraglobulin, the wall of a blood- 

 vessel has 110 such effect. The blood will remain fluid for 

 days in the veins of a sheep's trotter got from the butcher, 

 and yet will coagulate immediately when the veins are ripped 

 open with scissors (Lister). We do not know the explanation 

 of this, and we do not know to what the formation of fibriiiogeii 

 from albumen in the liquor sanguinis is due ; but what has 

 been said is sufficient to show that coagulation is not a vital 

 process, as was once supposed, but is a change of a chemical 

 description. 



80. The red corpuscles consist of a firm stroma with a sub- 

 stance in solution, which is partly composed of the para- 

 globulin already mentioned, but principally of a coloured 

 substance, hcemoglobin, which is an albuminoid with the 

 property of being crystallizable, the form of crystal varying 

 in different animals. The colouring matter can be entirely 

 separated from the albuminoid, but not without chemical 

 change, the product obtained being termed insoluble hcematin, 

 a substance remarkably distinguished by yielding more than 

 1 2 per cent, pure oxide of iron when burned. Iron is known 

 in medicine as a most powerful tonic in debility caused by 

 impoverishment or loss of blood, and this is in some measure 

 explained by the consideration that for the production of 

 blood, it is an essential ingredient. 



Haemoglobin is principally remarkable as the substance 

 which gives to the blood its power of absorbing oxygen. 



81. Blood contains in its composition an amount of gas, 

 which, when liberated, is nearly equal to half the volume of 

 liquid from which it has been set free. This gas can be ex- 

 tracted by means of the air pump, part of it easily, and the 

 rest with the aid of warmth. It contains a small quantity of 

 nitrogen, probably introduced in the lungs from the ex- 

 ternal aii-, in accordance with ordinary physical laws, and 

 not of any physiological importance. But the great bulk 

 of the gas consists of carbonic acid and oxygen, which vary 

 in their proportion in different parts of the circulation ; the 

 carbonic acid being, however, always in much larger volurue 

 than the oxygen. 



