CHAP, iv.] THE VASCULAR MECHANISM. 239 



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

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

 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 Development of the Normal Beat. 



133. The heart of a mammal or of a warm blooded animal 

 generally ceases to beat within a few minutes after being removed 

 from the body in the ordinary way, the hearts of newly-born 

 animals continuing however to beat for a longer time than those 

 of adults. Hence, though by special precautions and by means of 

 an artificial circulation of blood, an isolated mammalian heart may 

 be preserved in a pulsating condition for a much longer time, our 

 knowledge of the exact nature and of the causes of the cardiac 

 beat is as yet very largely based on the study of the hearts of 

 cold blooded animals, which will continue to beat for hours, or 

 under favourable circumstances even for days, after they have 

 been removed from the body with only ordinary care. We have 

 reason to think that the mechanism by which the beat is carried 

 on varies in some of its secondary features in different kinds of 

 animals : that the hearts, for instance, of the eel, the snake, the 

 tortoise and the frog, differ in some minor details of behaviour, 

 both from each other and from those of the bird and of the mammal ; 

 but we may, at first at all events, take the heart of the frog as 

 illustrating the main and important truths concerning the causes 

 and mechanism of the beat. 



