174 THE BLOOD AND ITS CIRCULATION 



plasma into the lymph; thus a slow current is maintained from the lymph 

 spaces into lymph tubes. On its course the lymph passes through many lymph 

 glands. In these glands some impurities appear to be removed and colorless 

 corpuscles made. The lymph ultimately passes into a large tube, the thoracic 

 (tho-ras'ik) duct, which leads upward near the ventral side of the spinal column, 

 and empties into the large subclavian vein in the left side of the neck. Another 

 smaller lymph duct enters the right subclavian vein. 



The Lacteals. — We have already found that part of the digested food 

 (chiefly sugars, amino-acids, salts, and water) is absorbed directly into the 

 blood through the walls of the villi and carried to the liver. Fat, however, is 

 passed into the spaces in the central part of the villi, and from there into other 

 spaces between the tissues, known as the lacteals. The lacteals carry the fats 

 into the blood by way of the thoracic duct. The lacteals and lymph vessels 

 thus have in part the same course. It will be thus seen that lymph at different 

 parts of its course would have a very different composition. 



The Effect of Exercise on the Circulation. — It is a fact familiar 

 to all that the heart beats more violently and quickly when we are 

 doing hard work than when we are resting. Count your own pulse 

 when sitting quietly, and then again after some brisk exercise in 

 the gymnasium. The average heart beat will be raised from about 

 72 beats a minute to between 90 and 100 a minute. Exercise in 

 moderation is of undoubted value, because it sends more blood to 

 parts of the body where increased oxidation is taking place as the 

 result of the exercise. The best forms of exercise are those which 

 give work to as many muscles as possible — walking, out-of-door 

 sports, any exercise that is not violent. Exercise should not be 

 attempted immediately after eating, as this causes a withdrawal of 

 blood from the digestive tract to the muscles of the body. Neither 

 should exercise be continued after becoming tired, as poisons are 

 then formed in the muscles, which cause the feeling we call fatigue. 

 Overdoing in any sport or game is dangerous. Fatigue is a signal 

 to rest. Obey it ! Remember that extra work given to the heart 

 by extreme exercise may injure it, causing possible trouble with 

 the valves. Older people and those who through excessive use 

 of stimulants or tobacco, or through overeating have developed 

 arteriosdero' sis or hardening of the arteries need to be especially 

 careful. ''A man is as young as his arteries," because the har- 

 dening of the wall raises the blood pressure, and if the inelastic 

 artery wall breaks, due to overexercise, death may result through 

 apoplexy. 



