816 PHYSIOLOGY 



fact that the shape of the granule, which differs from one species to another, 

 is the same whether the cell be derived from the blood or the bone- marrow. 

 The intermediate or transitional cell may be derived either from the 

 lymphocyte or from a myelocyte. In many cases of leukaemia the myelocyte 

 passes into the blood in large numbers without undergoing the changes 

 necessary to convert it into the typical blood-cell. We find then mono- 

 nuclear cells which are either free from granules or contain eosinophile or 

 basophile granules. 



FUNCTIONS OF THE LEUCOCYTES 



PHAGOCYTOSIS. We have seen that the leucocytes from whatever 

 animal they be taken present two phenomena, viz. that of amoeboid move- 

 ment and that of ingesting foreign particles which may be presented to 

 them. On account of this power of eating up foreign particles they are 

 frequently spoken of as ' phagocytes,' in this respect resembling unicellular 

 organisms and the undifferentiated cells of many kinds of tissue. All the 

 phenomena connected with the process of inflammation in higher animals 

 are directed to the assemblage of leucocytes at the spot which is the seat 

 of injury or of infection, so that they may devour and remove either the 

 injured tissue or the invading micro-organisms. This process plays there- 

 fore an important part in determining the immunity of any animal against 

 infection ; though in the higher animals it is assisted by a number of other 

 mechanisms directed towards the same end, which we shall have to discuss 

 in a subsequent chapter. The use of phagocytosis is not, however, confined 

 to the protection of the organism against infection. Wherever any effete 

 or dead tissue has to be cleared away, whether as the result of injury or 

 in the course of metamorphosis of organs, the leucocytes play an important 

 part. Thus in the great rearrangement of tissues which occurs in the 

 larval state of insects, the removal of the muscle fibres which are no longer 

 required is effected by the accumulation of phagocytes around them. The 

 phagocytes may send processes into the muscle substance, which dissolve 

 this tissue and then take it up. The absorption of the tail of the tadpole 

 is effected in the same way by means of phagocytes. In mammals, including 

 man, the moulding of the long bone which occurs in the process of growth 

 is effected by continual and coincident processes of absorption and new 

 formation of bone. The absorption is carried out by means of special 

 phagocytes formed by the aggregation of a number of leucocytes, the 

 well-known ' giant cells ' or myeloplaxes which form so prominent a 

 constituent of bone-marrow. 



The blood- corpuscles represent the wandering phagocytes of the body. 

 There are fixed phagocytes of which the myeloplaxes just mentioned may 

 be regarded as a type. Other members of this class are the endothelial 

 cells (Kupfer's ' Sternzellen ') which line the capillaries of the liver. If a 

 suspension of carmine or of micro-organisms be injected into the blood- 

 stream these endothelial cells are found a little later to have taken up 

 large numbers of the foreign bodies. Under normal circumstances these 



