CHANNELS OF INFECTION 5 



result of the invasion of bacteria at present not listed among 

 the pathogenic microorganisms, and what is true in this regard 

 for bacteria, may be hypothetically applied to the higher fungi 

 and to the protozoa. 



§ 3. Channels of infection. There are a number of 

 ways by which microorganisms may be introduced into the 

 living tissues of the animal body. The more common of these 

 are as follows, namely : 



1. Through the digestive tract. Bacteria gain entrance 

 into the tissues from the digestive tract where they have been 

 brought with the food or water. It is not clear in all cases 

 how the invading organisms get into the tissues from the 

 intestine. It has been demonstrated that tubercle bacteria will 

 pass through the mucosa with fat globules in the process of 

 digestion and absorption. 



2. Through the respiratory tract. Bacteria are taken into 

 the lungs with the inhaled atmosphere. Pulmonary tuber- 

 cular affection is often brought about in this way. 



3. Through abrasions of the skin or intestinal mucosa. 

 The wide distribution of bacteria in nature renders it highly . 

 probable that in all wounds of the integument microorganisms 

 will reach the fresh tissues. They may come from the cutting 

 or tearing implement, the particles of dirt which may fall into 

 or upon the cut surface, or from the ducts of the glands of the 

 skin itself It may happen that the fresh tissues thus exposed 

 are infected with one or .several species of bacteria. It may 

 be that one or more of these species may be destroyed by the 

 living juices of the body or by the leucocytes, or again it is 

 possible that, from their saprophytic nature, they may not be 

 able to multiple in this new environment ; in either case the 

 infection is of no significance and clinically would not be 

 recognized. It may happen that only one species of the infect- 

 ing bacteria multiplies and produces the morbid changes. 

 This would be a single infection. If, however, two or more 

 species cooperate in the production of the lesions, it is called 

 a mixed infection. This term is often used to designate the 



