CIRCULATION 



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unite to form large ducts. These lymph ducts are provided, like the veins, 

 with valves. The pressure of the blood within the blood vessels forces 

 continually more plasma into the lymph ; thus a slow current is maintained 

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

 through many collections of gland cells, the hjmph 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 up- 

 ward near the ventral side of the spinal 

 column, and empties into the large sub- 

 clavian 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 carbohydrates, peptones, salts, 

 and water) is absorbed directly into the 

 blood through the walls of the villi and 

 carried to the liver. Some of the food, 

 however, especially fats, makes its way 

 into the spaces in the central part of the 

 villi, and from there into other loose 

 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 different 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 sud- 

 den 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 heart 

 beat. The arteries and veins are also under the control of the sympathetic 

 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 called the vasomotor nerves. These. nerves act upon the 

 muscles in the walls of the blood vessels. In this way, each vessel adapts 

 itself to the amount of blood in it at a given time. After a hearty meal, 



Lacteal system of mammal ; a, descend- 

 ing aorta, or principal artery; h, 

 thoracic duct; c, origin of lacteal 

 vessels; g, in the walls of the intes- 

 tine, d', e, mesentery, or membrane 

 attaching the intestine to walls of 

 the body; /, lacteal, or mesenteric, 

 glands. 



