516 GENERAL CONSIDERATIONS 



optimum temperature, and if not aquatic must have a certain amount of 

 moisture, all of which conditions should be added to those which make for 

 healthful hving. 



545. Comparison of Protozoan and Metazoan Cells. — The cells in the 

 body of a metazoan are related to the body fluids in the same manner as 

 are the one-celled organisms to an aqueous environment. Though 

 there are cells on the surface which are not surrounded by these fluids 

 they must be related in some way to them in case they are to remain 

 living. The deeper cells of the human epidermis, for instance, are in 

 contact with blood vessels and with lymph, but as they are carried toward 

 the surface by the multiplication of cells below them, they lose this 

 contact, become dead, change in form and composition, and are finally 

 cast off. The surface cells of other animals are protected by a cuticula 

 which they secrete, by slime, or in some other fashion. Within the body 

 the health of the individual cell rests on much the same conditions as 

 does the health of the one-celled organism. Each cell must be provided 

 with the proper kinds of food and in the proper amounts, must be freely 

 supplied with oxygen, and must have its waste quickly removed. Each 

 cell must also have the proper environment maintained, including an 

 appropriate temperature, especially in the case of warm-blooded animals. 

 Light is a factor in the health of most animals, as are also a great variety 

 of internal secretions, especially in the higher forms. Since the health 

 of a metazoan is necessarily the resultant of the health of the different 

 cells which make it up, anything that interferes with the health of any 

 of the cells of the body produces a condition of disease, though it may be 

 local in character. 



546. Conditions of Health. — From what has just been stated it is 

 clear that four conditions are necessary for the maintenance of health: 



1. Proper kinds and amounts oi food. 



2. Maintenance of normal metabolic activity. 



3. Prompt and complete elimination oj waste. 



4. A proper physical environment. 



547. Causes of Disease. — The causes of disease are not alone the 

 converse of the conditions of health, although this is true of the first 

 cause here enumerated. These causes may be named as follows: 



1. Wrong Living Conditions. — These conditions involve food, air, 

 sleep, exercise, metabohsm, elimination, and internal secretions. 



2. Inheritance. — In certain cases a disease may be acquired before 

 birth; this is true of syphilis. Such acquisition is not true inheritance. 

 In other cases, the situation involves not the passing on of the disease 

 but the passing on of a weakened constitution which predisposes the 

 individual of the next generation to that disease; this is true in the case of 

 tuberculosis. Such susceptibilities may be truly inherited through the 

 germ plasm. 



