CIRCULATION OF BLOOD AND OF LYMPH 159 



there is another method of modifying the amount of blood 

 received by the separate organs. If the only method of 

 increasing the amount of blood sent to any organ were 

 by accelerating the rate of the heart beat, thus increasing the 

 circulation all over the body, it would be as inconvenient as 

 if the only way of regulating the gas in a house were by 

 turning it on or off at the gas factory. The body is 

 therefore, supplied with a more elaborate system so that 

 at any moment any single organ may receive a greater 

 or less supply of blood than usual. 



All the small arteries, as well as the large ones, are en- 

 circled by muscles (Fig. 82), whose contraction causes a dimi- 

 nution in the size of the blood vessels. If the muscles relax, the 

 artery will become larger because of the pressure of the con- 

 tained blood. The capillaries also contract and expand in a 

 similar way. Certain parts of the brain and spinal cord are 

 connected by nerves with all of these muscles and thus con- 

 trol their size. These nerves are called the vaso-motor nerves, 

 and the muscles, the vaso-motor muscles. The muscles and 

 nerves, together with their nerve centers in the brain and 

 spinal cord, constitute the vaso-motor system. 



Two sets of vaso-motor nerves are connected with the blood 

 vessels; one of them tends to constrict the vessels, and the 

 nerves composing it are called the vaso-constrictor nerves. 

 This set is acting all the time, keeping most of the vessels 

 slightly tightened; not sufficiently to shut off the flow of blood 

 but enough to prevent them from becoming loose and flabby; 

 this set supplies especially the vessels of the skin and intes- 

 tine. The other set of vaso-motor nerves causes the vessels to 

 enlarge; they are called the vaso-dilator nerves and arc found 

 especially supplying vessels of the muscles and glands. 



Action of the Vaso-Motor System. A few examples of the 

 work of this system may make its functions clearer. Ws 

 often hear people say that one should not take severe or 

 rapid exercise soon after eating. Why is this so? 



