^^. 



242 THE HEART. 



very scanty covering of this membrane (3, 2), inasmuch as that vessel 

 enters the right auricle almost immediately after passing through the 

 diaphragm, and is only partially surrounded by a reflection of the peri- 

 cardium in the narrow interval between these parts. None of the 

 vessels, indeed, joining the heart, with the exception of the aorta and 

 pulmonary artery where they are united together, receive a complete 

 covering from the pericardium, or can be said to be entirely enveloped 

 by the sac. In structure the serous layer of the pericardium agrees 

 with that of serous membranes generally, exce]3t that n o apertures have 

 ,-__,as yet been discovered in it by which its cavity communicates with 

 lymphatic vessels. 



"Wlien the left pulmonary artery and subjacent pulmonary vein are separated, 

 a fold of the pericardium will be seen between them, which has been termed by 

 Marshall the " vestigial fold of the pericardium." It is formed by a duplicature 

 of the serous layer, including areolar and fatty tissue, together with l^lood-vessels, 

 and nerves, and is from half to three-quarters of an inch in length, and from 

 half to one inch deep. It extends from the left superior intercostal vein above 

 the pulmonary artery downwards to the side of the left auricle, where it is lost in 

 a narrow streak which crosses romid the lower left pulmonarj^ vein. This fold is 

 y a vestige of a Irff superior vena cava (duct of Cuvier) , which exists in early em- 

 bryonic life, (Marshall, '" On the development of the great anterior veins in 

 Man and Mammalia,'' Philosoph. Trans. 18.50.) 



The pericardium is in relation in front and behind with the mediastina and 

 their contents. Anteriorly also it is overlapped by the pleui-a3 and to some extent 

 by the lungs, except below, where it approaches the siu-face in the angular space 

 to the left of the lower piece of the sternum. At the sides it is in contact with 

 the ]3hrenic nerves, as well as with the pleuraj and their contained viscera. Its 

 relations to the diaphi-agm and great vessels have been already noticed. 



THE HEART. 



The heart is a hollow muscular organ, divided by a longitudinal sep- 

 tum into a right and a left half, each of Avhicli is again sul)divided by a 

 transverse constriction into two compartments, communicating with 

 each other, and named auricle and ventricle. Its general form is 

 that of a blunt cone. Enclosed, as before said, in the pericardium, it 

 is placed behind the sternum and the costal cartilages (fig. 174 ; on 

 page 254), the broader end, or base, by which it is attached, being 

 — directed upwards, backwards, and to the right, and extending from 

 the level of the fourth to that of the eighth dorsal vertebra ; the 

 apex downwards, forwards, and to the left. In the living subject 

 its stroke against the wall of the chest is felt in the space between 

 the cartilages of the fifth and sixth ribs, a little below and to 

 the inner side of the left mammilla (3f inches from middle line of 

 sternum and 1| inch below nipple) : in the dead body the apex is a 

 little higher than during life. The heart, therefore, has a very 

 oblique position in the chest, and projects farther into the left than 

 into the right half of the cavity. Its position is afiFected to a certain 

 extent by that of the body ; thus it comes more into contact with the 

 anterior wall of the chest when the body is in the prone ])osture or is 

 lying on the left side. In inspiration, on the other hand, when the 

 diaphragm sinks and the lungs expand, it recedes slightly from the 

 chest-wall. 



At its base the heart is attached to the great blood-vessels ; moreover, 



