1 5 2 Life and Health 



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of Certain _ nejry.es. Within the substance of the heart are 

 masses of nerve matter called ganglia. From these gan- 

 glia there proceed, at regular intervals, discharges of nerve 

 energy, some of which excite movement, while others seem 

 to restrain it. 



The heart is also subject to nerve influences from out- 

 side of itself. Two nerves are connected with the heart, 

 the pneumogastric and the sympathetic (sees. 332 and 342). 

 The former appears to be connected with the restraining 

 ganglia, the latter with the exciting ganglia, 

 v 232. The Nervous Control of the Blood Vessels. The tone 

 and caliber of the blood vessels are controlled by certain 

 vaso-motor nerves, which are distributed among the muscular 

 fibers of the walls. These nerves are governed from a 

 center in the medulla oblongata (sec. 330). 



Now the vaso-motor center may be excited to increased 

 activity by influences reaching it from various parts of the 

 body, or even from the brain itself. As a result, the nerves 

 are stimulated, and the vessels contract. Again, the normal 

 influence of the vaso-motor center may ba suspended for a 

 time by what is known as the inhibitory, or restraining effect. 

 The result is that the tone of the blood vessels becomes 

 diminished, and their channels widen. 



The effect of this action of the nervous system is to give 

 it a certain control over the circulation in particular parts. 

 Thus, though the force of the heart and the general average 

 blood pressure remain the same, the state of the circulation 

 may be very different in different parts of the body. 

 233. The Importance of this Nervous Control. This local 

 control over the circulation is of the utmost importance. 

 Thus, when an organ is at work it needs to be more richly 

 supplied with blood than when at rest. For example, when 

 the salivary glands need to secrete saliva, and the stomach 



