136 



THE HUMAN MECHANISM 



system of a country is instructive. As different persons and 

 different communities in any country make different prod- 

 ucts and have different needs, it becomes more and more 

 necessary that the means of communication between them 



be extensive and efficient. 

 Hence the remarkable 

 growth of the railroads, 

 or " common carriers," of 

 any country in which in- 

 dustrial development pro- 

 duces increasing division 

 of labor. 



The blood, which is thus 

 the common carrier first 

 between the various or- 

 gans and second between 

 each organ and the outer 

 environment, is the net 

 product of the united 

 work of all the organs: 

 from the alimentary canal 

 it receives water and the 

 products of digestion ; 

 from the lungs it receives 

 oxygen ; each organ con- 

 tributes its share of waste 

 products or of internal 

 secretion, while some in- 

 fluence the composition of 

 the blood by removing 

 from it certain things that it contains. 



2. The microscopic structure of the blood. Examined under 

 the microscope the blood is seen to consist of a liquid portion, 

 the plasma, crowded with small solid bodies, the corpuscles. 

 These are of two kinds : the red corpuscles biconcave disks 



FIG. 65. Structure of a drop of blood as 

 seen under the microscope 



Above are shown nine red corpuscles highly 

 magnified ; below, less highly magnified, 

 the appearance of the blood soon after being 

 drawn. Two white corpuscles are shown, 

 and the red corpuscles stick together, form- 

 ing " rouleaux." Size of red corpuscle, 7.7 n 

 wide, 2-4 /^ thick. Diameter of white cor- 

 puscle, 5-10 M. Number of red corpuscles, 

 4,500,000-5,000,000 per cubic millimeter; 

 number of white corpuscles, 4500-13,000 

 per cubic millimeter, according to the state 

 of digestion, etc. Surface area of all the 

 red corpuscles of the blood, 3000 square 

 meters (30,000 square feet or approximately 

 four times the size of a baseball diamond). 

 (!M, or micron 0.001 millimeter) 



