PHAGOCYTOSIS 245 



of the leucocytes is due to their paralysis and destruction by the 

 powerful toxins which are given off. Certainly since the use of 

 methods involving experiments on phagocytosis in vitro no valid 

 example of negative chemotaxis has been adduced, and it is highly 

 probable that leucocytes are attracted by all bacteria, whether 

 their toxins are mild or potent. In the former case the leucocytic 

 infiltration will be very obvious ; in the latter the cells will be first 

 paralyzed and then destroyed as soon as they reach an area in 

 which the toxin is present in a high degree of concentration. 



We must not assume that this property of being attracted by 

 bacteria is of any advantage to the leucocyte itself, arguing from 

 the fact that the free-swimming unicellular organisms are attracted 

 by food and oxygen. The leucocytes are in many cases attracted 

 into the infected area to their own undoing, and it must not be 

 forgotten that even in an inflammatory process which is mild in 

 nature and favourable in result the number of leucocytes which 

 may be killed in the conflict is enormous. The leucocytes are not 

 independent protozoa inhabiting the blood and tissues, but an 

 integral part of the organism. It is to the advantage of the latter 

 that the former should be attracted at once to the seat of invasion, 

 and hence the processes of evolution have led to the development 

 of this function in the nomadic cells of the body. These are 

 extraordinarily susceptible to chemotactic influences ; they seem 

 to be attracted by any deviation from the normal constitution of 

 the tissues and fluid : a slight injury, a haemorrhage, the presence 

 of a poison, of a foreign body of any sort or of any dead or useless 

 tissue, and the leucocytes are immediately attracted into the area 

 affected. The more we regard the process, the more we must 

 regard it as one of the most exquisite examples of means to ends 

 met with in the animal economy. 



Secondly, with regard to the nature of the cells which have the 

 power of acting as phagocytes. Of these the most important are 

 the leucocytes, and especially the polynuclear and large hyaline 

 cells of human blood. All the leucocytes, however, have phago- 

 cytic powers, as is well seen in opsonic estimations : the cells 

 which take up the fewest bacteria are the eosinophiles and the 

 small lymphocytes. The former are very deficient in this direc- 

 tion, and we may be certain that their main function is entirely 

 different. The lymphocytes, too, take up but a small number of 

 bacteria ; but when activated by a suitable haemopsonic serum, they 

 take up red blood-corpuscles in considerable numbers, and a 



