XVIl] 



BACTERIOLOGY, ANTISEPTICS AND DISEASE 



199 



These microbes are larger than bacteria, and are 

 in some cases larger than the red blood cor- 

 puscles, which, in the horse, measure less than 

 rcnnj i n - m diameter. 



Bacteria, as stated above, produce disease by 

 forming a poison known as a toxin. If the toxin 

 is produced outside the system and injected into 

 the blood, as in lockjaw (tetanus), it is called 

 an extra-cellular toxin ; if produced inside, 

 as in anthrax, it is called an intra-cellular toxin. 

 There are two varieties of bacteria forming the 

 intra-cellular toxin : those like anthrax, which 

 spread over the whole blood system ; and those 

 like tuberculosis, which locate in certain glands 

 and remain there. 



781. Bacteria vary in size, but the average 

 size of a bacillus is TWISTS- i n - m length and 

 TvoffT;" 1 ' m diameter. Thus it will be seen how 

 much* smaller they are than red blood corpuscles. 

 The reader might reasonably ask how animal 

 micro-organisms are distinguished from vege- 

 table. As a matter of fact this is a difficult 

 question to answer, because in some forms of so- 

 called vegetable microbes there is more motion 

 and more animal signs than there are in many 

 forms of animal microbes. For instance, the 

 spirillum bacterium, that is supposed to produce 

 syphilis, looks just like the little worm-like 

 animal (equiperdum trypanosoma mastigophora) 

 that produces maladie du coit in horses, wrongly 

 called horse syphilis for this reason. Yet the 

 former is a vegetable and the latter an animal. 

 Again the rod-shaped bacillus (vegetable) that 

 produces typhoid has numerous little wriggling 

 legs (flagella), and yet many of the trypanosomes 

 and other protozoa are far less like animals to 

 look at under a high-power microscope with, 

 say, 2,000 diameters magnification. 



The real difference between vegetable and 

 animal microbes seems to be that the vege- 

 table microbe digests its food externally, thus 

 liberating at once all the useless elements and 

 assimilating only those that it can make use of ; 

 while the animal microbe, like all animals, 

 digests its food internally, and afterwards assi- 

 milates the useful elements and excretes the 

 useless ones. 



782. Pathogenic animal microbes are also 

 distinguished from bacteria, yeasts, and fungi in 

 that they generally live in two distinct hosts 

 the one in which they produce the disease (e.g. 

 the horse, in the disease known as nagana, 

 common in South Africa) ; and the other in 

 which they generally breed (a fly or mosquito, 

 e.g. the tsetse fly). The peculiarity of these 

 flies is that they alone are able to reproduce 

 in their stomach these protozoa, and after a 

 certain period, when they bite an animal, they 

 inject into that animal some of the virulent 

 protozoa. So that most protozoa, unless in com- 

 pany with one particular kind of fly, are unable 

 to produce disease. Hence it will be seen that 



the way to get rid of these diseases is to destroy 

 the disease-producing fly or mosquito. This is 

 what is being done now in many parts of the 

 world, e.g. Malta, South Africa, and South 

 America. (See Sec. 640.) 



783. Some non-pathogenic bacteria (sapro- 

 phytes) produce certain poisons called ptomaines, 

 and it is possible for a sufficient quantity of these 

 to be produced in the intestinal tract to cause 

 harm to the animal or person. However, most 

 so-called ptomaine poisoning is now put down 

 to pathogenic bacteria and not to the production 

 of ptomaine poisons. The toxins produced by 

 pathogenic bacteria are of a most poisonous 

 nature. Here is a comparison with other 

 poisons : 



Fatal dose for adult man, strychnine . J grain 



,, cobra venom -,L fj 



,, tetanus toxin 



(lockjaw). -^ 



One of the peculiarities of toxins is that they 

 have the power of producing antitoxins when 

 injected into suitable animal bodies. This dis- 

 covery was one of great importance in the pre- 

 vention of disease. 



784. Some of the commonest diseases pro- 

 duced by bacteria are the following : tuber- 

 culosis, tetanus, anthrax, influenza, glanders, 

 pyasmia, septicaemia, diphtheria, typhoid, pneu- 

 monia, syphilis, gonorrhrea, etc. 



Although in some of the following diseases 

 the presence of certain bacteria has been sus- 

 pected, up to the present time the specific cause 

 has not been isolated : scarlet fever, mumps, 

 smallpox, measles, whooping cough, yellow 

 fever, spotted fever, foot-and-mouth disease, and 

 chicken-pox. 



785. How Microbes Enter the Internal 

 System. In Sec. 779 an idea is given of the 

 various ways in which bacteria enter the system. 

 It is possible, but very exceptional, for bacteria 

 to enter through hair follicles and sweat glands ; 

 but if the animal is healthy, the tissues under 

 the skin will prevent this invasion ; resistance is 

 also increased by the antiseptic properties of 

 lymph. 



Generally speaking, bacteria reach the walls 

 of the intestines, i.e. the mucous membrane, by 

 gaining entrance with the food eaten by the 

 animal. When there, they must actually get into 

 the circulation by entering in between the cells 

 of the tissue ; the healthy state of the tissue 

 affects this considerably. They can also get in 

 by passing in with globules of fat that pass 

 through little openings that receive these 

 globules. Non-fatty material enters the circula- 

 tion by a diffusive process known as osmosis, 

 which is very similar to the way water passes 

 through the porous parts of a filter. Thus, fat 

 is a decided assistance to the admission of 

 microbes, and, therefore, septic milk (milk con- 



