PHYSIOLOGY FOR BEGINNERS 



the restraining influence removed, beats more quickly and 

 more strongly. 



There are also other nerves passing to the heart from the 

 spinal cord, impulses reaching -the heart along which cause a 

 quickening and strengthening of the beat. These impulses are 

 also originated in the spinal bulb, whence they are sent down 

 the spinal cord and then out along the nerves mentioned to 

 the heart. Thus the need of the body for a more active or less 

 active blood supply is regulated by a part of the spinal bulb. 



Regulation of the Blood- Vessels. When the heart's 

 beat is quickened all parts of the body get more blood than 

 before ; when it is slowed all parts get less. Quickening of 

 the heart's beat cannot give more blood to one organ without 

 giving more to all other organs. But it constantly happens 

 that some organs are doing much more work and require more 

 blood when other organs are doing, but little and require less. 

 When digestion is going on the stomach and intestines 

 require more blood, when muscular work is being done the 

 muscles require more, when brain work, such as thinking, is 

 being done the brain requires more blood, and so on. This 

 is brought about by a regulation of the blood supply carried 

 out in each organ or tissue. If the blood-vessels of an organ 

 get narrower, the resistance to the passage of blood is increased, 

 and less blood goes through them, more going through the 

 unchanged vessels of other organs. If the blood-vessels of an 

 organ widen, they offer less resistance to the passage of the 

 blood, and more blood goes through them, less going through 

 the unchanged blood-vessels of other organs. The small blood- 

 vessels can in this way change their bore or calibre, and 

 thus the blood supply to an organ is diminished or in- 

 creased. These changes are called the constriction, narrow- 

 ing, and the dilation, widening, of a vessel. These terms are 

 not to be confused with the contraction and relaxation of a 

 muscle, or with the elastic expansion and elastic recoil of an 

 artery due to the pulse. 



How are the constriction and the dilation of a vessel brought 

 about ? The smaller arteries consist, as we have seen, of an 

 endothelium with plain muscular fibres wrapped round it. 

 When these plain muscular fibres contract, as they become 

 shorter they must of necessity grip the vessel, making its bore 



