THE BLOOD g 



of a haemolytic serum be destroyed by heating to 55 C, the agglu- 

 tinating substance remains, being relatively unaffected by heat. 



If an animal, say a rabbit, be injected with the blood-cells of 

 the dog, the serum of the rabbit, which normally has no effect on 

 the blood-cells of this animal, becomes powerfully haemolytic for 

 dog's corpuscles, and, further, it agglutinates for the corpuscles of 

 the dog. An agglutinin has, therefore, been experimentally pro- 

 duced in the serum of the rabbit. The phenomenon of agglutina- 

 tion is employed in bacteriology as an important aid to diagnosis. 



Precipitins. — If the serum of one animal be injected into 

 another of a different species, it is found that the serum of the 

 receiver is capable of causing a precipitate in the normal serum 

 of the donor ; for example, if a rabbit be injected with the serum 

 of a dog, the rabbit serum will in course of time produce a pre- 

 cipitate if added to dog's serum, but not if added to the serum of 

 any other animal. Such substances are known as precipitins, 

 and are employed for the purpose of identifying different bloods 

 and for other purposes. They may, for example, be used for the 

 determination of the flesh of different animals — horse-flesh, for 

 instance, when sold as beef ; for if a rabbit be injected with an 

 extract of horse-flesh, its serum will produce a precipitate with 

 extracts of horse-flesh, but not ox-flesh. 



Blood Platelets. — These may be seen in the circulating blood, 

 but more easily in blood which has been shed — certain small 

 colourless cells one quattgr the size of a red corpuscle, and usually 

 of a round or oval shape. In shed blood they agglutinate and 

 rapidly disintegrate, but under suitable conditions they may be 

 kept alive, when they exhibit amoeboid movements. At one time 

 they were regarded as disintegration products of the red cells, but 

 this view is no longer held, and it is probable they are distinct 

 cellular elements. Of their function little or nothing is known, but 

 that they play an important part in blood-clotting is undoubted. 



Haemoglobin is the red colouring matter of the blood, and is 

 remarkable for being one of the most complex substances in 

 organic chemistry. It contains the elements C, H, O, N, S, and 

 Fe. The molecule of haemoglobin is probably the largest of any 

 known substance which is capable of being crystallised. If, as 

 is most usually assumed, its molecule contains one atom of iron, 

 then, on this assumption, and from a knowledge of its percentage 

 composition, the molecular formula for the haemoglobin of horse- 

 blood may be represented as C 112 H 11 3 () N 214 S 2 Fe0 2 45, which is some 

 5,000 times that of a molecule of hydrogen. It has been supposed 

 that the size of the haemoglobin molecule is connected with the 

 heavy atom of iron which it has to support. The function of the 

 iron is closely connected with the power the pigment has of 

 combining with oxygen. 



