BACTERIA AND DISEASE; GENERAL CONSIDERATIONS 119 



the intestines. Intrathoracic inoculation is rarely practised. 

 Injection directly into the heart (intracardiac) may be successfully 

 practised if sufficient care is used. Inoculation by scarification 

 is accomplished by scraping off the outer layers of skin without 

 drawing blood and rubbing the organism on the surface moistened 

 by the exuded serum. Intracranial injections may be made by the 

 use of a trephine to penetrate the skull, when subdural inocula- 

 tions are made with the hypodermic. Intra-ocular injections 

 have sometimes been performed for the specific purpose of observ- 

 ing from day to day the development of lesions upon the iris 

 or in other tissues of the eye. Inoculation by inhalation is accomp- 

 lished by forcing the animal to breathe the organism in dust or as 

 a fine spray. Infection by way of the digestive tract is accomp- 

 lished by feeding or ingestion. 



Interrelationships of the Organism and the Body. Any organ- 

 ism which produces lesions or morbid changes of any kind in the 

 body is said to be pathogenic, or disease producing. By virulence 

 is meant the relative ability of organisms of different races to 

 produce disease; for example, one culture of the diphtheria bacillus 

 might produce a severe type of disease, while another might be 

 wholly unable to produce an infection except under the most favor- 

 able conditions. The former is said to be more virulent than the 

 latter. 



Effects of Pathogenic Bacteria on the Body. When bacteria infect 

 the body, they may remain at or near the site of infection; they 

 may spread through the tissues by direct growth, or be carried by 

 the lymph and blood to other parts of the body; they may multiply 

 in the blood or they may produce metastatic infections by becoming 

 localized in other parts of the body. Disease may be produced 

 by the action of poisons, either toxins or endotoxins, or possibly 

 mechanically. The following classification adapted from Muir 

 and Ritchie is useful in a consideration of the types of changes 

 brought about in the body by microorganisms. 



A. Tissue changes. 



1. Produced in the immediate vicinity of the bacteria, either 



at the primary lesion or at secondary foci, 

 (a) Those changes resulting directly . from damage, as 

 degeneration and necrosis. 



