CHAPTER XIX 



THE CIRCULATORY SYSTEM 



THE circulatory system 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, 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 the large lymph spaces contained in the serous membranes. 



The Heart. 



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

 in the chest between the right and left lungs (fig. 190), 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; one layer 

 envelops 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 (pericardial 

 fluid) to lubricate the two surfaces and enable them to glide over 

 each other smoothly during the movements of the heart. The 

 presence of numerous elastic fibres in the epicardium enables it to 

 follow without hindrance the changing shape of the heart itself ; but 

 the parietal layer of the pericardium appears to be inextensible, and 

 so it limits the dilatation of the heart. 



