CHAPTER XVIII 



THE CIRCULATORY SYSTEM 



THE circulatory system consists of the heart, the arteries, or vessels 

 that carry the blood fr-om 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, (1) 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 lies 

 in the chest between the right and left lungs (fig. 216), 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 

 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 situ- 

 ated 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 (pericardia! fluid) to lubricate the two surfaces 

 and enable them to glide over each other smoothly during the move- 

 ments of the heart. The presence of 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. 



The pericardium is a comparatively simple serous membrane, because the 

 organ it encloses is a single one of simple external form. All serous membranes 



