THE PLASMA 83 



circulation, delivers, so to speak, a message that more pancreatic 

 juice is required; the pancreas is thrown into a state of activity 

 and secretes the necessary juice. Another example is furnished by the 

 ovary. When the mother becomes pregnant, the corpus luteum in 

 the ovary elaborates a hormone which, passing into the maternal cir- 

 culation, causes the large development of mammary tissue which takes 

 place previous to the birth of the young. It has also been suggested 

 that the embryo elaborates a hormone which, passing into the maternal 

 blood, further stimulates the growth of the mammary gland. After 

 birth this hormone no longer circulates, since its source is removed; the 

 breast tissue is no longer built up, but, on the contrary, the lack of 

 hormone leads to a breaking down of this proliferated tissue with the 

 formation of the first milk necessary to the young which have been 

 born. 



It is questionable whether the plasma contains enzymes. It has 

 no sugar-destroying ferment, but probably contains a lipase, which 

 plays a part in the fat metabolism of the body. 



The plasma is greatly concerned with the defences of the body 

 against poisonous substances or living intruders. . It provides a medium 

 in which the white corpuscles live, and one of their functions is to 

 migrate to the scene of attack and, if possible, repel or kill invading 

 bacteria. But besides the leucocytes there are in the blood defensive 

 substances known as antitoxins, immune bodies. These enable the 

 body either actively to neutralize, or in other ways to render ineffec- 

 tive, the harmful substances which may effect an entrance. 



' Plasma may be most readily obtained by receiving horse's blood 

 into a cooled vessel and allowing it to stand. The corpuscles sink 

 and the plasma comes to the top. Blood which has been kept from 

 clotting by the addition of neutral salts may be centrifuged; " salted " 

 plasma is thus obtained. Sodium citrate or potassium oxalate may be 

 employed to decalcify the blood and prevent clotting. 



The plasma must be regarded as a living tissue. It is always to 

 be borne in mind that the substances, considered below, obtained by 

 submitting to analysis living tissues, are dead products. The con- 

 stitution of the living protoplasm is a complex the nature of which 

 is only surmised. 



The Proteins. On analysis 7 to 8 per cent, of proteins are obtained; 

 these can be separated into three by the method of salting out 

 fibrinogen, serum globulin, and serum albumin. The amount of 

 fibrinogen is very small, 0-2 to 0-7 per cent. Present in blood-plasma, 

 but not in the serum, it is this protein which is concerned in the process 

 of clotting. 



Serum globulin is precipitated by hah* -saturation with ammonium 

 sulphate or full saturation with magnesium sulphate. Serum albumin 

 requires full saturation with ammonium sulphate. Fibrinogen is 

 thrown down by half-saturation with sodium chloride. 



Serum globulin is soluble in dilute saline solutions, but not in 

 distilled water. It is therefore precipitated from solution when the 



