274 HISTOLOGY OF THE HEART. [BOOK i. 



2. Changes in the peripheral resistance, due to variations in 

 the calibre of the minute arteries, brought about by the agency of 

 their contractile muscular coats. These changes may be either 

 local, affecting a particular vascular area only, or general, affecting 

 all or nearly all the blood vessels of the body. 



These two classes of events are chiefly governed by the 

 nervous system. It is by means of the nervous system that the 

 heart's beat and the calibre of the minute arteries are brought 

 into relation with each other, and with almost every part of the 

 body. It is by means of the nervous system acting either on the 

 heart, or on the small arteries, or on both, that a change of 

 circumstances affecting either the whole or a part of the body is 

 met by compensating or regulative changes in the flow of blood. 

 It is by means of the nervous system that an organ has a more 

 full supply of blood when at work than when at rest, that the 

 tide of blood through the skin rises and ebbs with the rise and 

 fall of the temperature of the air, that the work of the heart is 

 tempered to meet the strain of overfull arteries, and that the 

 arterial gates open and shut as the force of the central pump 

 waxes and wanes. The study of these changes becomes therefore 

 to a large extent a study of nervous actions. 



The circulation may also be modified by events not belonging 

 to either of the above two classes. Thus, in this or that peripheral 

 area, changes in the capillary walls and the walls of the minute 

 arteries and veins may lead to an increase of the tendency of the 

 blood corpuscles to adhere to the vascular walls, and so, quite 

 apart from any change in the calibre of the blood vessels, may 

 lead to increase of the peripheral resistance. This is seen in an 

 extreme case in inflammation, but may possibly intervene to a less 

 extent in the ordinary condition of the circulation, and may also 

 be under the influence of the nervous system. Further, any 

 decided change in the quantity of blood actually in circulation 

 must also influence the working of the vascular mechanism. But 

 both these changes are unimportant compared with the other two 

 kinds of changes. Hence, the two most important problems for 

 us to study are, 1, how the nervous system regulates the beat of 

 the heart, and 2, how the nervous system regulates the calibre of 

 the blood vessels. We will first consider the former problem. 



The Histology of the Heart. 



151. It will be necessary now to take up certain points 

 concerning the minute structure of the heart, which we had 

 previously postponed ; and since much of our knowledge of the 

 nervous mechanism of the beat of the heart is derived from ex- 

 periments on the hearts of cold-blooded animals, more particularly 

 of the frog, it will be desirable to consider these as well as the 

 mammalian heart. 



