518 Animal Biology 



prevent, treat, and cure diseases by scaring the demons by terrifying 

 noises, using vile-tasting or -smelling medicines, by exorcising the demon, 

 by wearing charms, etc. (2) Humoral theory of Hippocrates (460-395 

 B.C.) in which the body was thought to consist of four humors: blood, 

 phlegm, black bile, and yellow bile. Disease was thought to ensue if too 

 much or too little of one or another were present. Bloodletting was a 

 common curative procedure. (3) Pythogenic theory of Murchison in 

 which this Englishman, of about one hundred years ago, contended that 

 diseases were due to dirt and filth. (4) Germ theory in which diseases 

 were considered to be caused by minute organisms. Many individuals 

 contributed their bit to this theory as the following will show: Leeuwen- 

 hoek (1632-1723) studied microorganisms with his crude microscope; 

 Fracastorius, in 1546, suggested that infectious diseases were due to a 

 living contagion; Plenciz, in 1764, theorized that each disease was caused 

 by a specific microbe; Davaine, in 1850, proved that anthrax of cattle 

 was due to rodlike organisms; Pasteur, in 1865, showed that the silk- 

 worm disease (pebrine) was due to protozoa; the German country doctor, 

 Robert Koch (1843-1910), perfected many techniques in bacteriology 

 and proved that the cause of tuberculosis is a bacterium, known today as 

 Mycobacterium, tuberculosis. 



The mere presence of microorganisms in air, food, milk, and water is 

 not sufficient to produce a disease in all instances. As stated above, cer- 

 tain conditions must be fulfilled before an infectious disease will develop. 

 Among the most important deterrents to disease production in man are 

 the various body defenses, as the following will show : 



1. Defenses (First line). — Skin — acting as a mechanical barrier; nose 

 — mucus collecting the organisms, cilia moving them toward the exterior, 

 the enzyme lysozyme destroying bacteria, sneezing, coughing; eyes — 

 washing organisms away mechanically by tears which also contain the 

 enzyme lysozyme; m,outh — mucous membrane acting as a mechanical 

 barrier; stomach — acid of gastric juice; intestine — mucous membrane 

 acting as a mechanical barrier, antagonistic action of other organisms 

 within the intestine; urethra — action of urine. 



2. Defenses (Second line). — (1) The production of inflammations in 

 which many of the organisms which have penetrated the deeper tissues 

 are trapped and destroyed (inflammations are usually characterized by 

 redness and swelling due to increased supplies of blood; temperature, 

 due to increased metabolic activity in the area; pain, due to the ab- 

 normal activities; pus formation in later stages due to the destruction of 

 microorganisms and tissues) ; (2) phagocytic action of certain white blood 



