STRUCTURE OF THE HUMAN BODY 



39 



corpuscles are less numerous but somewhat larger ( ^soo of an 

 inch in diameter), and possess the power of spontaneous movement 

 by thrusting out lobes of the semi-fluid substance (protoplasm) 

 which composes them. They are thus enabled to slowly crawl, 

 as it were, along the walls of the blood-vessels. In the middle 



of each such colourless 

 corpuscle is a particle of 

 somewhat different chemi- 

 cal nature, the nucleus, 

 and this is probably to be 

 regarded as a modifi- 

 cation of the substance 

 composing the rest of 

 the corpuscle. No such 



Fig. 14. A Drop of Blood," seen under a microscope magnify- 

 ing by 350 diameters 



a and b, red corpuscles, c, white corpuscle. 



Fig. 15. White Blood Corpuscle. Its suc- 

 cessive changes of shape. 



particle can be discerned in a human red corpuscle, which is 

 therefore said to be non-nucleated, a character which happens 

 to be one of classificatory importance. 



THE HEART AND BLOOD-SYSTEM. This very complicated 

 organ is in principle a force-pump possessing an internal set of 

 valves which only permit fluid to pass in certain directions. The 

 blood-vessels which carry blood to it are veins, while those which 

 carry it away are arteries. To understand the heart's action it 

 is necessary to consider the simplest case, that of a heart with 

 only one chamber or cavity (fig. 16). We may represent such 

 a heart by an india-rubber syringe with a tube at each end, one 

 for carrying fluid in and the other for carrying it out. There 

 must also be, at least at one end, an internal valve consisting 

 of a flap which readily permits fluid to pass in one direction 

 but resists its passage in the other. In such a simple model 

 the central swelling will represent the heart, while the tube 

 conveying fluid into it will play the part of a vein, and the 

 other tube the part of an artery. Propulsion of fluid will be 



