THE BLOOD 



If fresh blood be stirred briskly with a bunch of twigs, the 

 fibrin is formed very rapidly, and collects on the twigs, leaving 

 behind a red fluid called defibrinated blood (blood deprived of its 

 fibrin) which will not coagulate. Thus : 



Fig. ii2. Hed Corpuscles 

 arranged in Rouleaux. 



Blood . 



The coagulation of blood may be retarded, and even prevented, by keep- 

 ing it at a temperature below 40 F., or at a higher temperature than 120 F. 

 The addition of common salt, or contact with living tissues, will also prevent 

 this change. On the other hand, coagulation may be hastened by free access 

 to the air (as by exposure in a shallow vessel), by allowing the blood to remain 

 at perfect rest, or by bringing it in contact with foreign (non-living) matter. In 

 the last case, the coagulation is rapid in propor- 

 tion to the surface which is in contact with the 

 blood ; hence ' the reason for whipping with a 

 bunch of twigs. 



Under ordinary circumstances the coagulation 

 sets in before the corpuscles have time to subside 

 or settle to the bottom of the vessel ; but the red 

 corpuscles have a tendency to adhere to each other 

 when the blood is at rest, forming rolls or columns 

 resembling piles of coins, the piles being connected 

 with each other at their ends so as to form a kind 

 of network which sinks more quickly than would 

 single corpuscles. When this change takes place 

 rapidly, the red corpuscles subside before the fibrin 

 is formed. In this case a red clot is formed by fibrin and red corpuscles at 

 the bottom of the vessel, over which is a yellowish clot produced by fibrin 

 and colourless corpuscles, together with a few of the red variety, sufficient to 

 impart a buff-yellow tint. This upper layer is commonly known as the buffy 

 coat, and is often cup-shaped in form, owing to the great shrinking of the fibrin 

 on account of the absence of the network of red corpuscles. 



The colour of the blood is due to the presence of a nitrogenous sub- 

 stance, called haemoglobin, in the red corpuscles. This substance contains 

 a considerable proportion of iron oxide, and has the power of combining 

 with oxygen gas. As the blood flows through the capillaries surrounding the 

 air-cells of the lungs, the haemoglobin combines with oxygen from the air, 

 becoming of a bright scarlet colour. The blood thus changed in colour 

 returns to the heart, by which it is forced throughout the body. Then, on 

 passing through the tissues of various organs, and especially the muscles, the 

 haemoglobin gives up some of its oxygen, which seems to be held by only a 

 feeble chemical affinity, to the carbon and hydrogen of these tissues, forming 

 respectively carbonic acid gas and water. The haemoglobin then changes to a 

 dark purple or purple-black colour, and the blood again returns to the lungs 

 with the carbonic acid gas in solution. In the lungs this gas is given off 

 into the air and expired, while the haemoglobin receives a fresh supply of 

 oxygen from the inspired air. From the above description it will be readily 

 understood why the haemoglobin has been termed the oxygen carrier of the 

 blood. 



