92 THORAX 



of the valve of the foramen ovale is marked by one or more 

 small semilunar depressions situated between slender muscular 

 ridges. The portion of the septal wall which lies below and 

 posterior to these depressions forms the floor of the fossa 

 ovalis, and is the remains of the valve of the foramen ovale of 

 the foetus. 



The Orifices of the Left Atrium. The orifices of the left 

 atrium are the openings of the four pulmonary veins, which 

 convey to it the oxygenated blood from the lungs ; a number 

 of minute openings which are the mouths of the vence cordis 

 minima ; and the left atria-ventricular orifice through which 

 blood passes from the left atrium to the left ventricle. 



The openings of the pulmonary veins are situated in the 

 posterior wall, nearer the upper than the lower part, and close 

 to the lateral borders, two on each side. They are entirely 

 devoid of valves. The orifices of the venae cordis minimae, 

 which are scattered irregularly, are also valveless ; but the left 

 atrio-ventricular orifice, which lies in the lower part of the 

 anterior wall of the atrium, is guarded by a bicuspid valve, the 

 mitral valve, which has been described already (p. 79). This 

 orifice is smaller than the corresponding orifice on the right 

 side, and admits the tips of two fingers only. 



The Structure of the Walls of the Heart. The last step in the dissection 

 of the heart consists in the examination of the structure of its walls. On 

 the outside the walls are covered with the epicardium, which is the visceral 

 part of the serous pericardium ; and on the inside they are lined with the 

 smooth and glistening endocardium, which plays a large part in the 

 formation of the flaps of the valves, and is continuous, through the 

 orifices, with the inner coats of the arteries and veins. Between the 

 epicardium and the endocardium lies the muscular tissue of the heart, 

 which is termed the myocardium. The muscular fibres of the myo- 

 cardium are disposed in layers, in each of which the fibres take a special 

 direction. 



The arrangement of the various layers of the myocardium cannot be 

 displayed in an ordinary dissecting-room heart, in which the continuity of 

 the fibres has been destroyed by the incisions made to display the cavities, 

 but the arrangement of the layers is practically the same in the hearts of 

 all mammals. Therefore, for the purpose of studying the layers, the 

 dissector should obtain a sheep's heart. This should be filled with a 

 paste made of flour and water ; then it should be boiled for a quarter of an 

 hour. The boiling expands the paste, softens the connective tissue, and 

 hardens the muscular fibres. After the boiling is finished the heart should 

 be placed for a time in cold water. After it has cooled, first the epicardium 

 and then the muscular fibres should be gradually torn off. 



The atrial fibres are difficult to dissect. They consist of three groups : 

 (i) A superficial group running more or less transversely and common to 

 both atria. They are best marked near the coronary sulcus. (2) A deep 

 group special to each auricle. The extremities of these fibres are connected 



