ANGEIOLOGY. THE HEART. 297 



SECTION IV. ANGEIOLOGY. 



UNDER the name of Angeiology is included the descriptive anatomy of the 

 vascular system, consisting of the heart, blood-vessels, and absorbents. The 

 Jteart is the central organ of the circulation, and, although presenting a com- 

 plex structure and mechanism, may be regarded, when viewed with reference 

 to its development, as a curved and greatly altered blood-vessel. The 

 blood-vessels are of three kinds viz., the arteries, or ramifying vessels which 

 distribute the blood from the heart ; the capillaries, or network of simple 

 walled microscopic vessels, in which the blood is diffused through the 

 tissues ; and the veins, or vessels by which the blood is returned to the 

 heart. The absorbents are the small and delicate vessels which convey into 

 the circulation fluid material capable of being converted into blood, whether 

 derived directly from the food digested in the alimentary canal, or returned 

 from the tissues in which it has already played some part in the nutritive 

 processes. 



The double circulation. In the systemic circulation the blood is conveyed 

 from the left ventricle of the heart by the arteries to every part of the body, 

 and, having parted in the capillaries with a portion of its ingredients, and 

 undergone changes which render its purification necessary, it is returned by 

 the veins to the right side of the heart, which is distinct from that from 

 which it set out. The dark-coloured blood thus brought back to the right 

 side of the heart is conducted through the pulmonary circulation, being 

 propelled through the pulmonary artery by the right ventricle, undergoing 

 in the lungs a process of purification, in which it receives oxygen from the 

 air and parts with carbonic acid, and returning thence to the heart by the 

 pulmonary veins, again to enter the systemic circulation. 



THE HEART. 



RELATION TO SURROUNDING PARTS. 



The heart is situated in the thorax, between the two lungs, and, together 

 with the adjacent parts of the great vessels which convey blood to and from 

 it, is enclosed by a membranous covering, the pericardium. It is placed 

 behind the sternum and the costal cartilages, occupying a region of about 

 four inches in width, extending from the second intercostal space on the 

 right side to the fifth space on the left, and reaching considerably farther on 

 the left than on the right of the middle line. 



THE MEDIASTINUM. 



The greater part of the thorax is occupied by the lungs, each of which is 

 invested with a serous membrane, the pleura, which presents a parietal and 

 a visceral portion, and is continued "from the one portion to the other by 

 passing on the surface of the pericardium from the anterior and posterior 

 parts of the walls of the chest to the root of the lung. Thus the heart, 

 enclosed in the pericardium, is situated between the right and left pleural 

 sacs, and between the layers of an antero-posterior septum formed by the 

 united portions of the right and left pleurse, and known as the mediastinum. 



