THE CIRCULATION 211 



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 thicken- 

 ing 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 pulmonic 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 muscles extend from one to the other. 

 If the heart is boiled, the auricles, the aorta and the pul- 

 monary artery may be separated from the ventricles. This 

 is because boiling converts fibrous tissue to gelatine and 



