CH. xviii.] THE CIRCULATORY SYSTEM. 2O; 



CHAPTER XVIII. 



THE CIRCULATORY SYSTEM. 



THE circulatory system is the collection of organs which have 

 for their function, the circulation of the blood. It consists of the 

 heart, the arteries or vessels that carry the blood from the heart to 

 other parts of the body, the veins or vessels that carry the blood 

 back to the heart again, and the capillaries, a network of minute 

 tubes which connect the terminations of the smallest arteries to 

 the commencements of the smallest veins. We shall also have to 

 consider in connection with the circulatory system, (i) the 

 lymphatics, which are vessels that convey back the lymph (the 

 fluid which exudes through the thin walls of the blood-capillaries) 

 to the large veins near to their entrance into the heart, and (2) 

 the large lymph spaces contained in the serous membranes. 



The Heart. 



This is the great central pump of the circulatory system. It 

 is contained in the chest or thorax. It lies between the right 

 and left lungs (fig. 207), and is enclosed in a covering called 

 the pericardium. The pericardium is an instance of a serous 

 membrane. Like all serous membranes it consists of two layers ; 

 each consists of fibrous tissue containing a large amount of elastic 

 fibres ; one layer envelopes the heart and forms its outer covering 

 or epicardium ; this is the visceral layer of the pericardium ; the 

 other layer of the pericardium, called its parietal layer, is situated 

 at some little distance from the heart, being attached below to the 

 diaphragm, the partition between the thorax and the abdomen. 

 The visceral and parietal layers are continuous for a short distance 

 along the great vessels at the base of the heart, and so form a closed 

 sac. This sac is lined by endothelium ; in health it contains 

 just enough lymph to lubricate the two surfaces and enable 

 them to glide over each other smoothly during the movements 

 of the heart. The presence of elastic fibres in the epicardium 

 enables it to follow without hindrance the changing snaps 

 of the heart itself ; but the parietal layer of the pericardium 

 appears to be inextensible, and so it limits the dilatation of 

 the heart. 



