THE HEART. 131 



pleurse, arid resting upon the cordiform tendon of the diaphragm, in 

 the cavity of the thorax; it is surrounded by a membrane called the 

 pericardium. Its sha2oe is conoidal, its apex inclines to the left side, 

 touching the walls ot" the thorax, between the fifth and sixth ribs. 

 It is somewhat flattened upon that side which rests upon the tendon 

 of the diaphragm. It measures five inches and a half from its apex 

 to its base, three inches and a half in the diameter of its base, and 

 weighs about six or eight ounces. It contains /owr cavities, and per- 

 forms two functions, that of receiving the blood and throwing it into 

 the lungs, and that of receiving it again after it has been oxygenated, 

 and distributing it throughout the body. The auricles are receptacles, 

 and the ventricles propel the blood to the lungs and through the 

 system. 



The auricle and ventricle of the right side receive and propel the 

 venous blood into the lungs. The auricle and ventricle of the left 

 side receive and propel the arterial blood. 



The circidation of the blood is as follows : the ascending and de- 

 scending vena cava empty the venous blood into the right auricle; 

 from here it passes to the right ventricle, through an opening pro- 

 tected by a valve opening downwards; from the right ventricle it is 

 propelled through the pulmonary artery, which divides into two 

 branches, to the lungs ; after it has been subjected to the influence 

 of the respiratory process, it is brought from the lungs, by four pul- 

 monary veins into the left auricle. The left auricle has an opening 

 into the lefl ventricle, protected by a valve opening downwards, and 

 from the left ventricle it passes into the aorta, thence to be dis- 

 tributed throughout the body. 



The right auricle^ is an irregularly-shaped cavity, somewhat 

 oblong and cuboidal; anteriorly it has a convexity which is called 

 its sinus, superiorly there is an elongated process resemblii*g the ear 

 of an animal, whence the term auricle. Its walls are thin, and com- 

 posed of muscular fibres, which from their parallel arrangement re- 

 semble the teeth of a comb, and hence are called Tiiusculi pectinati. 

 The superior^ and inferior vena cavaD enter the auricle from behind, 

 and between their orifices there is an elevation called tubercidum 

 Loweri. On the septum or partition between this and the left auricle, 

 is a depression called fossa ovalis, where formerly existed the fora- 

 men ovale of foetal life ; the ring or edge surrounding it, is called the 

 anmdus ovalis, from which there extends downwards to the inferior 

 semi-circumference of the ascending cava, a crescentic doubling of 

 the lining membrane, which is the remains of the Eustachian valve. 

 The coronary veins open into this cavity, and their orifice is pro- 

 tected by the valve of Thebesius. The opening to the ventricle, called 

 the ostium venosmn, is circular and surrounded by a dense white 

 line. 



