[nicholls] unsolved PROBLEMS OF IMMUNITY 259 



been gained, the duties of protection devolve upon the blood and certain 

 of the tissue cells and juices. The cells chiefly concerned in the com- 

 bat against the aggressive agents are the polymorphonuclear leucocytes 

 and hyaline cells of the blood, the lining endothelial cells of blood- 

 vessels, lymph-spaces, and serous sacs, and to a less extent those of 

 connective tissue. There is evidence, too, for thinking that the blood 

 plasma contains substances that are inimical to the growth of infective 

 micro-organisms. There is another factor, also, to be considered, 

 namely, the lymphatic channels and the lymph-nodes. The lymphatic 

 channels in the large majority of cases determine the line of invasion 

 of the offending substances, and the nodes interpose a barrier in the 

 shape of a filter, not merely a mechanical filter, but more than this, 

 a vital filter, the cells of which have, to some extent, the power of 

 fixing the deleterious agents and neutralizing their effects. From one 

 ,point of view this is a decided gain, but from another, is harmful in 

 that the presence of an extensive and intercommunicating lymph sys- 

 tem renders the systemic dissemination of the offending substances more 

 easy. 



The bodily temperature appears also to be of moment. Many 

 pathogenic germs can flourish only within a comparatively narrow tem- 

 perature range. Outside of these limits their growth is inhibited or 

 stopped altogether. This probably explains in part why it is that 

 micro-organisms that are injurious to man are innocuous to the lower 

 animals. Tn the coldblooded animals we have not only a low temper- 

 ature, but in some cases a varying temperature, a condition of things 

 that is highly inimical to many germs. In certain of the higher 

 vertebrates the temperature is two or three degrees higher than it is 

 in man. It would seem probable, also, though upon this point we 

 have little or no information, that the plasma of human blood differs 

 considerably from that of the lower animals in the matter of the con- 

 tained salts and their relative proportions, and we have experimental 

 evidence to show that a comparatively slight alteration in the percentage 

 of the chemical substances contained in nutrient media is sufficient to 

 materially inhibit the growth of bacteria. We may find in this an 

 explanation of the variations in the susceptibility of various species of 

 animals. 



To give an adequate explanation of the variation in susceptibility 

 in different races of the same species is extremely difficult. We loiow 

 that many primitive races, living remote from civilization, such as 

 the Eskimo, Icelanders, and North American Indians, are extremely 

 susceptible to the ravages of infectious diseases like tuberculosis, small- 

 pox and syphilis. The comparative immunity of other peoples is 



