CIKCULATION 



gradually broadening bed, the greatest breadth of the channel 

 being reached in the capillaries. From this point the channel 

 gradually narrows until the heart is reached. 



Hence the blood stream is very rapid in the arteries where 

 the channel is narrow, and very sluggish in the capillaries where 

 the channel is wide, so that in them plenty of time is allowed 

 for exchanges between the blood and the tissues. 



II. THE CENTRAL PUMP THE HEART 

 A. Structure 



A very simple form of heart exists in the ascidians. At 

 one point on a large vessel there is a thickening in the wall 

 composed of non-striped muscular fibres. A contraction is 

 seen to pass from one end of this to the other at frequent 

 regular intervals, thus forcing the fluid through the vessels. 

 The embryonic heart in man has a similar structure. 



In the snail and cuttle-fish, in addition to the contracting 

 muscular thickening, there is also a thin-walled receiving 

 chamber into which the blood flows before it is expelled 

 onwards. The heart is thus composed of two chambers. 



1st. A receiving chamber the auricle. 



2nd. An expelling chamber the ventricle. 



In fish the heart has a similar structure. But in lung- 

 bearing animals a more complex arrangement is required, 

 and a double heart is found, one concerned with the pro- 

 pulsion of blood to the system generally, and hence called 

 the systemic heart ; one propelling blood to the lungs, and 

 hence called the pulmonie heart. In mammals, the former 

 chamber is on the left side, the latter on the right. Each 

 consists of a receiving and expelling chamber an auricle 

 and a ventricle. 



The walls of these chambers are essentially muscular ; but 

 this muscular layer, or myocardium, lies between two fibrous 

 layers, the pericardium and the endocardium. 



The musculature of the auricles is separate from that of 

 the ventricles, but some fibres more like ordinary visceral 

 fibres than cardiac muscle extend from one to the other. 

 This band of His' plays a most important part in conducting 

 contraction started in the auricles to the ventricles. If the 



