CHAPTER IV 



ANATOMICAL AND PHYSIOLOGICAL FACTORS IN 



INFECTION AND RESISTANCE OF THE 



INDIA^CDUAL 



When the opportunity occurs for an infectious agent to come into 

 contact with the animal body, it is said that the animal is exposed 

 to infection. The portal of entry, dosag-e, and virulence of the 

 patliogen, as weU as the anatomical and phj'siological condition of 

 the host are factors determining whether entrance is gained to the 

 body and whether disease results. 



The significance of many of these factors has been described in 

 the preceding pages. Attention has been called to the presence of 

 myriads of potentially pathogenic micro-organisms present on the 

 body surfaces awaiting an opportunity to invade the tissues. 



The first line of defense against invasion of the tissues is the 

 epithelial coverings of the body and the various secretions which 

 bathe them. The second line of defense consists of cells of the 

 reticulo-endothelial system present in the skin or mucous mem- 

 branes and in the lymph glands which drain these areas. When 

 one considers the central nervous system, he finds that the choroid 

 plexus seems to act as a barrier between the general circulation 

 and the circulation of the brain and cord. 



The skin, with its outer insoluble, keratinized epidermis, its multi- 

 ple layers of stratified squamous epithelium, and rich capillary 

 blood supply, offers a substantial barrier to bacterial invasion. 

 Williams (1941) says that the nature and low^ pH of the outer 

 layer discourage growth and invasion of bacteria. In his opinion 

 the resurfacing mechanism of the body is important in defense. 



While tlie mucous membrane of the buccal cavity, as well as 

 those of the nose and throat are knoAvn to harbor a rich bacterial 

 flora, they are protected to a certain extent from contact with bac- 

 teria by a film of mucus. Secretions of the submaxillary and 

 parotid glands enter the buccal cavity through their respective 

 ducts while secretions from the bronchi and trachea are carried 

 upward to the mouth by cilia of the trachea. The phj^siologic vigor 

 and integrity of the mucous membrane are dependent, to a large 



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