Circulation, Assimilation and Excretion 423 



Again, the rate of blood flow may be increased too much by 

 too violent exercise. Many young athletes have injured their 

 hearts for life by too great exertion in athletics. 



Sluggish circulation tends to weaken the power of the body 

 to resist colds. Habitual care of the health, including the taking 

 of outdoor exercise, sufficient food and' plenty of rest are the 

 preventives and the only safeguards against colds. Prevention is 

 worth more than cure. 



Ductless Glands. Ductless glands are glands that pass sub- 

 stances, known as hormones, not through ducts, but directly into 

 the blood, which diffuses them throughout the body. In this 

 respect they differ from such glands as the salivary glands, 

 which discharge their principal products through ducts directly 

 into the parts where they function. Among these glands are the 

 thyroid, the spleen and the adrenals. 



The thyroid glands are located one on each side of the 

 esophagus just below the larynx. Although they apparently do 

 not form a part of any system, they are important because of 

 their influence on the life processes of the body. The secretion 

 which they provide in some way affects the nutrition of the body. 

 In the absence of the secretion, a child, as he grows, may develop 

 a misshapen body with weak limbs and a dull brain. 



The spleen is near the lower wall of the stomach and is always 

 well supplied with blood. Its use is not fully known and it can be 

 removed from the body without serious results. 



The adrenal glands are two in number, each about the size 

 of a walnut, and are situated one in front of each kidney. They 

 influence the action of the muscles in the walls of the arteries 

 by means of their secretion, and if destroyed cause a serious 

 disease in the blood. 



Coordination. The function of these internal secretions, 

 which are chemical, appears to be to coordinate the work of 

 different parts of the body, that is, to influence the various parts 

 of the body machine so that they will work in harmony and move 

 at the right time and in the right order. 



It has been discovered that certain glands which ordinarily 

 discharge substances into various parts of the body through ducts 



