THE GENERAL ANIMAL FUNCTIONS 87 



will produce profound effects on the well-being of other groups of 

 cells far removed from them. For example, products of the 

 sex and other glands get into the blood and produce growth in 

 the voice box and vocal cords in man and in fact all the outward 

 differences in the bodies of males and females. If the thyroid 

 is diseased or removed, so that its product is no longer poured 

 into the blood, profound changes take place in certain other 

 structures in wholly different parts of the body. If the healthy 

 thjrroid tissue of another individual is grafted at any point in 

 the body of such a diseased individual the latter will become 

 normal again. These various organs secrete substances (hor- 

 mones or endocrines) into the blood which control activity else- 

 where. The proper balance of these chemical substances goes 

 far to coordinate and make normal the various human functions 

 and development. 



Indeed we are coming more and more to understand that 

 every cell in the body changes the blood in some degree for all 

 the other cells of the body. It takes from the blood as it passes, 

 and at the same time pours out into the blood the by-products 

 of its own living. For example, we know that overwork causes 

 fatigue products to be poured into the blood with profound 

 effects upon the animal. These substances may even be in- 

 jected into the blood of a rested animal and bring about symp- 

 toms of fatigue. We must think of the blood as more than just 

 a means of connecting lungs and intestine and kidneys with one 

 another and with the general system. The life of every cell in 

 the body is conditioned by the make-up of this medium whose 

 composition is determined by every cell in the body. We are 

 just beginning to appreciate its full meaning ; but we know that 

 this chemical integration of the body is profoundly important. 



c. The Coordinating Reintegration. — In addition to the phys- 

 ical and chemical unity, tlie parts of the body must work in 

 harmony and instantly. When food is eaten the digestive 

 glands must become active. When the body exercises the oxy- 

 gen supply must become more rapid. When an organ acts it 

 must have an increased blood supply. When we undertake to 

 walk, scores of muscles must contract just enough and in the 

 right relation to other muscles. When external conditions 



