THE BLOOD. 257 



of the new-born rat. A small quantity of this tissue moistened with normal 

 saline and examined microscopically with suitable powers will show large 

 numbers of plaques within the blood-vessels. 



As to the origin of the blood platelets there has been much difference of 

 opinion. Many theories have been proposed, none of which have been 

 accepted. As a result of long continued observations Wright has recently 

 published results which make it probable that they are fragments or detached 

 portions of the cytoplasm of giant cells, megakaryocytes, found in the 

 marrow of the bones. The cytoplasm is prolonged into pseudopod-like 

 processes which become detached, and as they are in close relation to 

 the blood channels they are soon taken up and carried into the blood 

 of the general circulation when they are known as blood platelets or 

 plaques. 



The function of the blood-plaques is unknown, but it has been surmised 

 that in some way they are, like the leukocytes, concerned in the coagula- 

 tion of the blood. Whenever they are diminished in number, as in purpura 

 and hemophilia, coagulation takes place very slowly. 



THE TOTAL QUANTITY OF THE BLOOD; ITS GENERAL 

 COMPOSITION. 



The determination of the total quantity of the blood in an animal is best 

 made by the chromometric method, somewhat modified at present, of 

 Welcker. This consists, first, in bleeding an animal, collecting all the blood 

 it yields, and weighing it; second, in washing out the vessels with a normal 

 saline solution until the fluid comes from the veins clear and free from blood; 

 third, in mincing the tissues of the body, after removal of the contents of the 

 alimentary canal, soaking them in water for twenty-four hours, and then 

 expressing them. All the washings are collected and weighed. A given 

 volume of the normal defibrinated blood, treated with carbon monoxid so as 

 to give it uniform color, is then diluted with water until its tint is identical 

 with that of the washings similarly treated with carbon monoxid. From the 

 quantity of water necessary to dilute the blood the quantity of blood in the 

 washings is readily determined. The animal having been previously 

 weighed and the weight of the contents of the alimentary canal deducted, the 

 ratio of the total weight of the blood to the weight of the body at once be- 

 comes apparent. By this method it has been shown that the ratio of blood 

 to body- weight in a human adult is 1:13; in an infant, 1:19; in a dog, 1:13; 

 in a cat, 1:21. 



The more recent investigations of Haldane and Smith and of Plesch 

 with the employment of a different method make it probable that the ratio 

 is approximately 1:19. Thus a man weighing 70 kilos would have 3684 

 grams of blood. 



The amount of blood in the different organs has been determined by 

 ligating the blood-vessels in the living animal, removing the organ, and after 

 allowing the blood -to escape subjecting the tissues to the chromometric 

 methods described above. According to Ranke, the volume of the blood is 

 distributed as follows: Heart, lungs, arteries, and veins, J; liver, J; muscles, J; 

 other organs, J. 

 17 



