CHAPTER LXVIII 



ANIMAL ORGANISMS IN HEALTH AND DISEASE 



A very practical application of animal biology is in the development 

 of an understanding of health and disease. The principles involved are 

 of general application to all animals, but since we are more concerned with 

 health and disease in man and more is known of human diseases than of 

 those of other animals, any discussion of this subject will inevitably have 

 a strong human emphasis. 



541. Definitions. — If an organism is in perfect adjustment to its 

 external environment, and in case it is a metazoan, if the cells which com- 

 pose the body are perfectly adjusted to the conditions within it, then 

 theoretically the organism will be carrying on the activities of life w^th the 

 maximum degree of ease and effectiveness. Such a condition could be 

 referred to as one of ideal health. If, on the other hand, there is any 

 departure from that condition so as to interfere with the carrying on of 

 such activities even in slight degree, the condition might be termed one of 

 disease. Perfect adjustment, however, either external or internal, is 

 rarely if ever encountered. To all organisms life involves a constant 

 struggle to reach an adjustment sufficient to avoid a serious interference 

 with the performance of bodily functions. Therefore, for practical 

 purposes health is defined as the existence of such a condition of the organ- 

 ism as permits it to carry on all functions in a normal fashion, though it 

 may be not to a maximum of effectiveness. Disease, on the contrary, 

 is the existence of a condition which interferes with such normal functional 

 activity. With reference to ourselves, we ordinarily overlook little 

 troubles in various parts of the body, such as mild headaches, slight dis- 

 turbances of digestion, and other small ills which appear to us of little 

 consequence, and consider ourselves well in spite of them. 



542. Health in a Protozoan. — The protozoan, a frequent environment 

 of which is the water in which it lives, needs for healthful living food 

 of the right kind and in the necessary amount. It also needs to be 

 perfectly adjusted to the environment about it so that there will be no 

 interference with the interchange of material between itself and the water, 

 including respiration and the prompt elimination of waste. Given these 

 conditions the presumption is that normal metabolic activity will be 

 carried on and that the protozoan will continue to live a healthful life. 

 Most one-celled organisms also need a certain amount of light, have an 



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