THE BLOOD AND ITS CIRCULATION 



32.^ 



directly into the blood throup;h the walls of the villi and carried to 



the liver. Fat, however, is passed into the spaces in th<' central 



part of the villi, and from there into otlier spactes Ixitween the 



tissues, known as the ladcals. 



The lacteals carry the fats into 



the blood by way of the thoracic 



duct. 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. — Al- 

 though the muscles of the heart 

 contract and relax without our be- 

 ing 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 

 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, 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 shouUl 

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



The lymph vessels ; the dark spots are 

 lymph glands : lac, lacteals ; re, tho- 

 racic duct. 



