30 A MANUAL OF PHYSIOLOGY 



it throws upon that peculiar and specific response which the body 

 makes to the presence of foreign cells or juices, and which con- 

 stitutes an attempt to render itself ' immune ' to them. 



In each case the specific antibody seems to be produced in response 

 to the presence of some particular constituent of the foreign cell. 

 The substances which on injection give rise to antibodies are spoken 

 of as antigens. In the case of the erythrocytes there is evidence 

 that the antigens (both the haemolysinogen, which causes the pro- 

 duction of specific amboceptor, and the agglutininogen, the substance 

 which gives rise to specific agglutinin) are lipoids, or are so closely 

 associated with the lipoids of the corpuscles that they are extracted 

 by the same solvents. Thus ethereal extracts of erythrocytes cause 

 the production of haemolysin and agglutinin, just as the entire cor- 

 puscles do. Indeed, the response of the animal body to the presence 

 of foreign substances is so catholic, and at the same time so exquisitely 

 specific, that many artificially isolated proteins, even those of vege- 

 table origin, after as careful purification as possible, occasion, when 

 injected, the production of antibodies which will precipitate from a 

 solution only the variety of protein injected. 



Precipitins. When the serum of one animal is injected into 

 another of a different group, the serum of the latter acquires 

 the property of causing a precipitate in the normal serum of 

 animals of the same group as that whose serum was injected, 

 but not in the serum of any other kind of animal. Thus, if 

 human blood or serum is repeatedly injected into a rabbit, 

 the serum of the rabbit will cause a precipitate in diluted human 

 blood or serum, but not in the blood or serum of other animals, 

 except that of monkeys, where a slight reaction may be obtained. 

 The specific bodies which cause the precipitation are termed 

 precipitins. The phenomenon has been made the basis of a 

 method of distinguishing human blood for forensic purposes. 



Since changes begin in the blood as soon as it is shed, having 

 for their outcome clotting or coagulation, we have to gather 

 from the composition of the stable factors of clotted blood, or 

 of blood which has been artificially prevented from clotting, 

 some notion of the composition of the unaltered fluid as it 

 circulates within the vessels. The first step, therefore, in the 

 study of the chemistry of blood is the study of coagulation. 



Coagulation of the Blood. When blood is shed, its viscidity 

 soon begins to increase, and after an interval, varying with the 

 kind of blood, the temperature of the air, and other conditions, 

 but in man seldom exceeding ten, or falling below three, minutes, 

 it sets into a firm jelly. This jelly gradually shrinks and 

 squeezes out a straw-coloured liquid, the serum. Under the 

 microscope the serum is seen to contain few or no red corpuscles ; 

 these are nearly all in the clot, entangled in the meshes of a kind 

 of network of fine fibrils composed of fibrin. In uncoagulated 

 blood no such fibrils are present ; they have accordingly been 



