1 90 HUMAN BIOLOGY 



environment. Sounds occur which we do not hear, and 

 hght waves, notably the ultraviolet, strike us which we 

 cannot see, yet influence us profoundly. 



The cells of our lungs are adapted to the taking-up of 

 oxygen and the giving-off" of carbon dioxide. The cells of the 

 ahmentary tract take in foods and those of the kidney 

 throw out waste products. Muscle cells enable us to move and 

 work. As we have intimated, some cells are stationary and 

 others highly motile. Sperms in search of eggs to fertihze 

 may travel relatively long distances. Leucocytes in the blood 

 stream are washed hither and thither but may actively 

 migrate through the vessel walls to attack invading bacteria 

 and other harmful agencies. They may be likened to the 

 policemen; the fat cells (which store potential energy), to 

 the bankers; the muscle cells, to the laborers; the gland 

 cells to the manufacturers; and so on, while the nerve cells 

 form an hereditary ruling class. 



Cells, like individuals, through their special tempera- 

 mental activities affect their surroundings. In the body 

 they are bathed in fluids the character of which they modify. 

 These fluids are comparable to the atmosphere, often 

 polluted, in which we live. Important physiological changes 

 take place in this watery environment so that the activity 

 of the body cannot be regarded merely as the unrelated 

 sum total of those of the component cells. An element of 

 integration is added through which elements, themselves 

 diff"erent, by association may produce something wholly 

 new, the character of which could not have been forseen, 

 just as oxygen and hydrogen, two gases, on combination 

 make a different substance, water. The medium about the 

 cells may not remain fluid, but may be converted by the 

 cells into various substances, among which is bone, without 

 which we would be spineless creatures indeed. Cells of like 

 character are grouped into what we call tissues, such as 

 cartilage. The tissues, in turn, are often combined to form 

 organs, as, for example, the thyroid gland, which on 

 enlargement produces a goiter. 



The behavior of cells is dependent upon their ancestry, 

 their environment (or training) and their age. Thus the 

 blood-forming organs produce blood cells and the deeper 



