THE CLOTTING OF BLOOD. 61 



sons overworked and confined within doors. In such cases 

 the best remedies are open-air exercise and good food. 



Summary. Practically the composition of the blood may 

 be thus stated: It consists of (1) plastna, consisting of watery 

 solutions of serum -albumin, paraglobulin, fibrinogen, sodi- 

 um and other salts, and extractives of which the most con- 

 stant are urea, kreatin, and grape-sugar; (2) red corpuscles, 

 containing rather more than half their weight of water, the 

 remainder being mainly haemoglobin, other proteids, and pot- 

 ash salts; (3) white corpuscles, consisting of water, various 

 proteids, glycogen, and potash salts; (4) the platelets ; (5) 

 gases, partly dissolved in the plasma or combined with its 

 sodium salts, and partly combined (oxygen) with the haemo- 

 globin of the red corpuscles. 



Quantity of Blood. The total amount of blood in the 

 Body is difficult of accurate determination. It is about -fa 

 of the whole weight of the Body, so the quantity in a man 

 weighing 75 kilos (165 Ibs.) is about 5.8 kilos (12.7 Ibs.). Of 

 this at any given moment about one fourth would be found in 

 the heart, lungs and larger blood-vessels; and equal quantities 

 in the vessels of the liver, and in those of the muscles which 

 move the skeleton; while the remaining fourth is distributed 

 among the remaining parts of the Body. 



The Origin and Fate of the Blood-corpuscles. The white 

 blood-corpuscles vary so rapidly and frequently in number in 

 the blood that they must be constantly in process of altera- 

 tion or removal, and formation ; their number is largely in- 

 creased after taking food, even more than that of the red, so 

 that their proportion to the red rises, from 1 to 1000 during 

 fasting, to 1 to 250 or 300 after a meal. This increase is 

 mainly due to increased flow of lymph at this time through 

 the lymphatics of the alimentary canal which have much 

 lymphoid tissue on their course; and, as already pointed out, 

 lymph-corpuscles are constantly multiplying in this tissue 

 and are gathered from it by the lymph, to be poured into the 

 blood (see also Chap. XXIII). Migrated pale corpuscles of 

 the blood and the leucocytes of the lymph retain many of the 

 characters of undifferentiated and unspecialized embryonic 

 cells; and there is some evidence that they may develop new 

 tissues in the repair of injured parts. 



Amphioxus, the lowest undoubted vertebrate animal (see 

 Zoology), possesses only colorless corpuscles in its blood. 



