134 HANDBOOK OF PHYSIOLOGY. 



increase, and vice versa, the force of the cardiac beat, although the fre- 

 quency is diminished as the blood-pressure rises. (3) The quantity (and 

 quality?) of the blood contained in its chambers, too, has an influence 

 upon its systole, and within normal limits the larger the quantity the 

 stronger the contraction, Eapidity of systole does not of necessity indi- 

 cate strength, as two weak contractions often do no more work than a 

 strong and prolonged one. (4) In order that the heart may do its maxi- 

 mum work, it must be allowed free space to act; for if obstructed in its 

 action by mechanical outside pressure, as by an excess of fluid within 

 the pericardium, such as is produced by inflammation, or by an over- 

 loaded stomach, or the like, the pulsations become irregular and feeble. 



Functions of the Arteries. 



The External Coat. The external coat forms a strong and tough 

 investment, which, though capable of extension, appears principally de- 

 signed to strengthen the arteries and to guard against their excessive 

 distention by the force of the heart's action. It is this coat which alone 

 prevents the complete severance of an artery when a ligature is tightly 

 applied ; the internal and middle coats being divided. In it, too, the 

 little vasa vasorum (p. 109) find a suitable tissue in which to subdivide 

 for the supply of the arterial coats. 



The Elastic Tissue. The purpose of the elastic tissue, which 

 enters so largely into the formation of all the coats of the arteries, is, 

 (a) to guard the arteries from the suddenly exerted pressure to which 

 they are subjected at each contraction of the ventricles. In every such 

 contraction, the contents of the ventricles are forced into the arteries 

 more quickly than they can be discharged into and through the capilla- 

 ries. The blood therefore, being, for an instant, resisted in its onward 

 course, a part of the force with which it was impelled is directed against 

 the sides of the arteries ; under this force their elastic walls dilate, 

 stretching enough to receive the blood, and as they stretch, becoming 

 more tense and more resisting. Thus, by yielding, they break the shock of 

 the force impelling the blood. On the subsidence of the pressure, when 

 the ventricles cease contracting, the arteries are able, by the same elas- 

 ticity, to resume their former calibre, (b.) It equalizes the current of 

 the blood by maintaining pressure on it in the arteries during the 

 periods at which the ventricles are at rest or dilating. If the arteries 

 had been rigid tubes, the blood, instead of flowing, as it does, in a con- 

 stant stream, would have been propelled through the arterial system in 

 a series of jerks corresponding to the ventricular contractions, with in- 

 tervals of almost complete rest during the inaction of the ventricles. 

 But in the actual condition of the arteries, the force of the successive 

 contractions of the ventricles is expended partly in the direct propulsion 



