THE VASCULAE MECHANISM. 191 



tricuspid valves, the systole of the ventricle makes itself distinctly felt in 

 the great veins ; and a distention travelling backward from the heart 

 becomes very visible in the veins of the neck. This is sometimes spoken of 

 as a venous pulse. 



Variations of pressure in the great veins, due to the respiratory move- 

 ments, are also sometimes spoken of as a venous pulse ; the nature of these 

 variations will be explained in treating of respiration. 



THE REGULATION AND ADAPTATION OF THE VASCULAR MECHANISM. 



The Regulation of the Beat of the Heart. 



139. So far the facts with which we have had to deal, with the excep- 

 tion of the heart's beat itself, have been simply physical facts. All the 

 essential phenomena which we have studied may be reproduced on a dead 

 model. Such an unvarying mechanical vascular system would, however, 

 be useless to a living body whose actions were at all complicated. The 

 prominent feature of a living mechanism is the power of adapting itself to 

 changes in its internal and external circumstances. 



The vascular mechanism in all animals in which it is present is capable 

 of local and general modifications, adapting it to local and general changes 

 of circumstance. These modifications fall into two great classes : 



1. Changes in the heart's beat. These, being central, have, of course, a 

 general effect; they influence or may influence the whole body. 



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

 the minute arteries, brought about by the agency of their contractile mus- 

 cular coats. These changes may be either local, affecting a particular vas- 

 cular area only, or general, affecting all or nearly all the bloodvessels 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 cir- 

 cumstances affecting either the whole or a part of the body is met by com- 

 pensating 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 bloodvessels, 

 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, 



