232 HANDBOOK OF PHTSIOLOGY. 



And may on the other hand be diminished: 



(c.) If the force and frequency of the heart's beats be diminished, 

 and if the peripheral resistance be (I) unchanged, or be (2) in- 

 creased. 



(d.) If the force and frequency of the heart's beats be unchanged, 

 and the peripheral resistance be increased. 



When the force and frequency of the heart's contractions are in- 

 creased and at the same time the peripheral resistance is increased, the 

 flow may be increased, diminished, or unchanged, according as either (<f 

 the two factors, one of which tends to increase the flow, and the other 

 to diminish it, is more markedly increased, or if they are balanced. The 

 complemented proposition is also true, that the flow may be increased, 

 diminished, or unchanged, when the force and frequency of the heart's 

 contractions are diminished, and the peripheral resistance is diminished. 



The conditions of increased flow and of increase of blood pressure 

 are not the same. Indeed, the greatest blood-flow may occur when the 

 blood pressure is low, i.e., when the peripheral resistance is diminished 

 and the heart's beat is increased or is unchanged. In fact there is only 

 one condition in which increased blood-flow is accompanied by increased 

 blood-pressure, viz., when the heart's beat is increased and the periphe- 

 ral resistance is unchanged. 



It will be necessary now to consider (a) the ways in which the force 

 and frequency of the heart's beats are regulated / and also (b) the ways in 

 which tlie peripheral resistance is increased or diminished. We shall 

 afterward be in a better position to discuss the variations of blood- 

 pressure produced by different combinations of cardiac and arterial 

 alterations. 



(a.) The force and frequency of the contractions of the heart 

 may be considered to depend upon : 



1. The properties and condition of the heart-muscle itself; 



2. The influence of the central nervous system; 



3. The amount of the blood passing into the heart's cavities ; 



4. The amount of pressure to be overcome. 



5. The coronary circulation. 



Each of these factors must be considered seriatim. 



1. The properties of the heart-muscle. It has already been pointed 

 out that in structure the muscular fibres of the heart differ from skeletal 

 muscle on the one hand, and from uustriped muscle on the other, occupy- 

 ing, as it were, an intermediate position between the two varieties. The 

 heart-muscle, however, possesses a property which is not possessed by skele- 

 tal muscle, or by unstriped muscle to such a degree, namely, the property 

 of rhythmical contractility. The property of rhythmic contraction 



