■Ill THE PRESENT EVOLUTION OB' MAN — PHYSICAL 



surface. The " local lesion " caused by them may be, 

 and often is, trivial in depth and extent, yet the whole 

 organism may bo powerfully affected. Moreover, it is 

 not necessarily the tissues in the near neighbourhood of 

 the lesion tliat are most affected, but frequently tissues 

 far distant. For instance, the microbes of tetanus 

 cause in the neighbourhood of the lesion — e.g. a wound 

 in the hand or foot, to which they have gained entrance — 

 an inflammation of no great extent, and indistinguishable 

 from infl.amniations due to other causes ; but notwith- 

 standing that the local lesion is so slight, the muscles of 

 the body, beginning with those of the head and neck, and 

 passing to those of the trunk and limbs, are thrown 

 into violent contraction, so that the sufferer, resting 

 perhaps on his head and heels only, lies rigid, bent like 

 a bow by the spasm of his strong posterior muscles. 

 The main, the distinctive pathological effect, is therefore 

 not due to any direct action of the micro-organisms, but 

 to a soluble poison (toxin) produced by them, which 

 enters the blood stream at the point of the lesion, and 

 pervades and poisons the whole body, acting probably, 

 like strychnine, chiefly on certain nerve-cells which 

 have for their functions the control of the muscles. 

 Similarly as regards most other zymotic diseases, 

 whether the micro-organisms enter the blood stream or 

 not, the j^athological effects produced are directly 

 traceable, not to the micro-organisms, but to their 

 toxins, as is proved by the fact that if these micro- 

 organisms are cultivated, as in many instances they 

 have been, in a suitable artificial medium outside the 

 body, and then killed or separated — e. g. by filtration — 

 from the medium, the latter, since it contains the 

 sj)ecific toxins, when injected into the body produces 

 all the distinctive symptoms of the disease. 



Now, it has been found in the case of various 

 zymotic diseases of which one attack confers immunity, 



