230 HUMAN DISEASES 



harmless types are unable to do so, points to the fact that the 

 former have special means of offence. When studying heredity 

 in relation to disease, it is necessary to know something of these 

 means of offence and defence, otherwise we are liable to fall into 

 the error, which vitiates so much speculation on the subject, of 

 mistaking somatic modifications for germinal changes. 



383. Just as the different species of microbes have different 

 means of entering the body, so. when once the entrance is affected, 

 they tend to select different parts or tissues of the body for their 

 habitation. Some, as those of malaria and probably of all 

 'general' diseases such as influenza, smallpox, measles, and 

 scarlet fever, inhabit the blood or lymph. They multiply with 

 great rapidity, and penetrate to every region. On the other hand, 

 tuberculosis especially affects the lungs and lymphatic glands, 

 more rarely the membranes of the brain, the joints, and the skin. 

 The microbes of diphtheria are usually limited to the mucous 

 membrane of the upper air passages, those of pneumonia multiply 

 in the lungs, those of cholera, enteric fever, dysentery, and epidemic 

 diarrhoea in the bowels. The microbes of tetanus are limited 

 to the small area surrounding the wound by which they have 

 gained entrance. 



384. Though the area of body invaded by the microbes of a 

 disease may be very small, yet in many cases the sufferer becomes 

 extremely ill. Thus in tetanus he is thrown into convulsions, 

 which resemble those produced by strychnine, and which indicate 

 that his spinal column is in some way poisoned. We may search 

 the cord in vain for bacilli. Again, it is possible to grow several 

 species of pathogenic microbes in broth or other nutrient fluid 

 outside the body. If this medium is filtered so that it is entirely 

 free from living organisms, and then injected into the body, the 

 symptoms of the disease may follow. It is evident, therefore, that 

 at least some species of microbes elaborate poisons, or toxins as 

 they are technically termed, which they set free in the medium 

 surrounding them. In tetanus and diphtheria the toxins enter the 

 blood stream at the small area affected, and so produce the 

 symptoms of poisoning. Toxins, like pepsin and trypsin, which 

 are digestive secretions from the cells of the alimentary tract of 

 the higher animals, are evidently ferments; that is, substances 

 which in some unknown way bring about chemical changes in 

 certain other substances when the latter are exposed to their 

 action. 



385. The cells of the multicellular organisms are specialized 



