104 ELEMENTARY PHYSIOLOGY less. 



corpuscles, rounded, colourless particles may, but with 

 difficulty, be made out as existing in blood. These 

 are known as "blood platelets." They are extremely 

 minute, not much wider than the thickness of a red 

 corpuscle, and usually disappear as soon as blood is removed 

 from the body. But so little is known about them that 

 we must not do more than simply draw attention to their 

 existence. 



5. The Origin and Fate of the Corpuscles.— The 

 exact number of both red and colourless corpuscles present 

 in the blood varies a good deal from time to time ; and 

 there is reason to think that both kinds of corpuscles are 

 continually being destroyed. But since, on the whole, 

 the average numlier of each kind of corpuscle is main- 

 tained during healthy life, it is evident that new cor- 

 puscles must be continually forming to take the place of 

 those which have disappeared. 



Our knowledge of the origin of the red corpuscles is 

 somewhat indefinite ; there is, however, no doubt that 

 in the Jidult the chief .seat of their formation lies in that 

 marrow found in the cavities of bones which, from being 

 very plentifully su{)plied with blood-vessels, is known as 

 red marrO"W. The majority of colourless corpuscles, 

 those which exhibit amu3l)oid movement and granular 

 cell-bodies also have their origin in the red marrow of the 

 bones. There is some doubt as to whether the cells 

 which give rise to red corpuscles in the marrow are similar 

 to ordinary white corpuscles, or are a particular kind of 

 cell ; and the question has not as yet been definitely de- 

 cided as to how the mammalian red corpuscle comes to 

 have no nucleus, although formed in or from cells which 

 are themselves nucleated. The lymphocytes originate 

 in the lymphatic glands and other similar structures, 

 are then passed along the lymphatic vessels into the 

 blood. 



Apart from what is known as to the disappearance of 

 white corpuscles from the blocjd by migration through the 



