THE BLOOD AND ITS CIRCULATION 377 



maintained from the lymph spaces toward the veins. On its course the 

 lymph passes through many collections of gland cells, the 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 duct, which flows 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 

 ^ein. 



;The Lacteals. — We ha^e already found that part of the digested food 

 (chiefly carbohydrates, peptones, salts, and water) is absorbed directly 

 into the blood through the walls of the viUi and carried to the liver. Pat, 

 howe\-er, 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 form the most direct course for the fats to reach the blood. The 

 lacteals and lymph vessels have in part the same course. It will be thus 

 seen that lymph at different parts of its course would have a very diflier- 

 ent composition. 



'/The Nervous Control of the Heart and Blood Vessels. — Although 

 the muscles of the heart contract and relax without our being able to stop 

 them or force them to go faster, yet in cases of sudden fright, or after a 

 sudden blow, the heart may stop beating for a short interval. This shows 

 that the heart is under the control of the nervous system. Two sets of 

 nerve fibers, both of which are connected with the central nervous system, 

 pass to the heart. One set of fibers accelerates, the other slows or inhibits, 

 the heartbeat. The arteries and veins are also under the control of the 

 s\Tnpathetic nervous system. This allows of a change in the diameter 

 of the blood vessels. Thus, blushing is due to a sudden rush of blood to 

 the surface of the body, caused by an expansion of the blood vessels at 

 the surface. The blood vessels of the body are always full of blood. This 

 results from an automatic regulation of the diameter of the blood tubes by 

 a part of the nervous system caUed the vasomotor nerves. These nerves 

 act upon the muscles in the walls of the blood vessels. In this way, each 

 \'essel adapts itself to the amount of blood in it at a given time. After 

 a hearty meal, a large supply of blood is needed in the walls of the stomach 

 and intestines. At this time, the arteries going to this region are dilated 

 so as to receive an extra supply. When the brain performs hard work, 

 blood is supplied in the same manner to that region. Hence, one should 

 not study or do mental work immediately after a hearty meal, for blood 

 will he drawn away to the brain, leaving the digestive tract with an in- 

 sufficient supply. Indigestion may foUow as a result. 



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 



