OLD AGE AND DEATH 373 



In concrete illustration of old age phenomena, let us 

 take Metchnikoff s account of the hair turning grey in man, 

 and his account of the state of a very old captive parrot. 

 Turning grey can hardly be called more than a pheno- 

 mena of senescence ; the parrot showed senility, so that 

 the two cases do not quite illustrate the contrast that 

 usually obtains between man and beast in old age. 



In almost all animals, from sponges to mammals, there 

 is, within or apart from the vascular fluids, a bodyguard of 

 wandering amoeboid cells, technically called phagocytes. 

 It is well known that they perform many functions both 

 in health and in disease : they form a bodyguard, struggling 

 with invading microbes, surrounding and engulfing irritant 

 particles, transporting useful material, helping to repair 

 wounds, aiding in the regeneration of lost parts, and in 

 metamorphosis, and so on. To the pathologist, " phago- 

 cytosis 5 ' has become a word of comfort, and perhaps there 

 has been some exaggeration of its importance. But no one 

 can deny that the phagocytes play an important and 

 versatile part in the vital economy. 



If we hold a hair against a strong light we see at once a 

 distinction between a lighter central portion the medulla, 

 and a darker peripheral portion the cortex. Both 

 parts are built up of cells, but the medullary part is the 

 more living of the two. Metchnikoff discovered that 

 the disappearance of pigment from hair beginning to 

 turn grey is due to the intervention of phagocytes. These 

 amoeboid cells appear in the medullary part of the hair 

 and make their way out into the cortex, where they absorb 

 the pigment granules, which they then remove from the 

 hair. In a hair beginning to turn white, there are many 

 phagocytes laden with pigment especially about the root 

 of the hair. In absolutely white hair the colour of which 

 ;s due to gas bubbles there are no phagocytes with 



