184 CIRCULATORY SYSTEM. 



apex. The right auricle and the right ventricle, are the two 

 cavities which are nearest to the right side of the body, while 

 the left auricle and the left ventricle are the two cavities near- 

 est to the left side. It will, however, be understood, from the 

 general observations already made, that the relative situation 

 of these cavities is such, that the right ones arc in front <.f the 

 others, and present obliquely forwards to the right side, while 

 those on the left side look obliquely backwards to the left side. 

 This position of the heart makes it encroach more upon the left 

 cavity of the thorax than it does on the right; from which 

 cause its pulsations may be very easilv distinguished where the 

 left ribs join their cartilages, while on the right side of the ster- 

 num there is scarcely ever a perceptible pulsation. Being 

 placed between the right and the left pleura, in the mediasti- 

 num, it is surrounded by its own proper capsule called the pe- 

 ricardium. 



The Pericardium is covered on its sides by the pleurae, and 

 reposes on the tendinous centre of the diaphragm, to which it 

 adheres by close compact cellular substance, particularly at its 

 periphery. When the latter attachment is cut through, a sepa- 

 ration of the remainder is easily effected. Behind, the pericar- 

 dium is opposed to the bronchia and the oesophagus. 



The pericardium does not adhere to the heart, except at the 

 base of the latter; it is, therefore, a loose capsule in, by far, the 

 greater part of its extent. It not only surrounds the heart, but 

 also the roots of the large arteries and veins connected with it. 

 Thus, it includes the aorta, as high up as the great vessels pro- 

 ceeding from its arch; from the latter, it passes to the trunk of 

 the pulmonary artery, and also includes it, causing the aorta 

 and the pulmonary artery to lie close together. The posterior 

 face of these vessels is not covered so high up as the anterior 

 face. The pericardium also invests the descending vena cava 

 for an inch above its junction with the right auricle: it likewise 

 invests the trunks of the pulmonary veins, and the ascending 

 cava as it rises above the diaphragm. The pouches which it 

 forms at the base of the heart, in passing from one of these ves- 

 sels to another, are the comua of some anatomists. It cannot 

 be considered as pierced for the passage of these vessels, but is 

 lost insensibly on their parietes; being continued into the cellu- 



