284 BACTERIA AND DISEASE 



within a range of tubercular infection, and the specific bacilli of 

 tubercle gain access to the lungs. The result will be a case of 

 consumption more or less acute according to environment and 

 treatment. 



Channels of Infection 



The channels of infection by which organisms gain the vantage- 

 ground afforded by the depressed tissues are various, and next to 

 the maintenance of resistant tissues they call for most attention 

 from the physician and surgeon. It is in this field of preventive 

 medicine — that is to say, preventing infective matter from entering 

 the tissues at all — that science has triumphed in recent years. It 

 is, in short, applied bacteriology. 



1. Pure Heredity. — This term is to be understood in this connec- 

 tion as concerned with actual transmission of germs of disease from 

 the mother to the child in utero. That such conveyance may occur 

 is admitted, but it is certainly not frequent, nor is bacterial disease 

 widely spread by this means. The transmission of tendency (diathesis) 

 is, of course, another matter, and there can be little doubt that ante- 

 natal conditions exert an influence on bacterial diseases of infancy. 



2. Inoculation, or inserting virus directly through a broken sur- 

 face of skin, is a method of producing diseases in animals commonly 

 used in experimental work. Such inoculations may be subcutaneous, 

 intravenous, intracerebral, intraperitoneal, etc. In the natural pro- 

 duction of disease, inoculation is also a not uncommon channel of 

 infection. Injuries of the skin caused by instruments, gunshot 

 wounds, broken glass or china, etc., may serve as the point of intro- 

 duction of specific virus. Tetanus is commonly an inoculated disease. 

 Malaria must now also be so considered. Local tuberculosis is not 

 infrequently produced by inoculation through a broken skin surface. 



3. Contagion indicates that a disease is transmitted by personal 

 contact, through unbroken skua surfaces. Small-pox, measles, ring- 

 worm, and other diseases may be thus contracted. It is not unlikely 

 that as our knowledge grows, the diseases to be defined as spread by 

 contagion will become less. 



4. The Alimentary System. — Many diseases are spread by the 

 consumption of infected food or water, and in children the sucking 

 of dirty objects may introduce germs of disease into the alimentary 

 canal. Milk, cream, butter, cheese, ice-cream, oysters, shell-fish, 

 meats of various kinds, vegetables, water-cress, ice, and a large variety 

 of foods, have been the means of introducing pathogenic organisms 

 into the body, and in this way enteric fever, cholera, dysentery, and a 

 large number of acute and chronic diseases are originated. Water- 

 borne disease furnishes a large percentage of such cases. 



5. The Respiratory Tract. — The air may become infected with 



