94 ELEMENTS OF PHYSIOLOGY 



layer gives smoothness and prevents friction. Why are 

 these three properties necessary to blood vessels ? 



155. Why the Blood Vessels must be Elastic. The 

 aorta and its branches are full of blood all the time. 

 When the left ventricle with its great muscular walls con- 

 tracts, the blood cannot move forward into the narrow 

 arteries and capillaries fast enough to make room for the 

 new supply so suddenly sent out of the ventricle. There- 

 fore the aorta becomes more than full. If a cup is full, it 

 cannot become " fuller" ; not so with an artery. The yellow 

 elastic fibers of its connective tissue allow it to expand as 

 a rubber hose does under pressure. The first part of the 

 aorta having expanded to receive the incoming blood, the 

 portion of the aorta just ahead of the expanded portion is 

 less tense, or tight, so the stretched elastic fibers contract 

 and force blood into it, expanding it in turn. Thus a 

 wave of expansion travels along the blood vessel. It is 

 called the pulse, and may be most easily felt in the wrists 

 and neck. The distended elastic walls exert pressure on 

 the blood in the arteries, and this presses some of the extra 

 blood out of them into the capillaries. As much blood as 

 is being pressed on into the capillaries is being thrown into 

 the aorta by the beat of the heart ; so that during life a 

 distension is always kept up, and the blood in the vessels 

 is always under pressure. Although the arteries may get 

 rid of the additional distension following each heart beat, 

 there is a normal distension that always remains. It has ex- 

 isted ever since life began, and will remain until the heart 

 ceases to beat. The pulse, therefore, is only an additional 

 distension following the contraction of the ventricle. 



You should not think that the muscular layer actively 

 contracts and helps to send along the pulse ; for the pulse 

 is simply the passive stretching and contracting of the 

 elastic tissue ; as a wave travels across a pond when a stone 

 is dropped into the water. The force of the pulse is 



