120 THORAX 



auricular appendage) is covered with musculi pectinati. In 

 a formalin hardened heart the dissectors will find a strong 

 muscular ridge which descends from the posterior margin of 

 the commencement of the cavity of the auricle along the 

 left border of the cavity of the atrium anterior to the orifices 

 of the left pulmonary veins, entirely concealing them from 

 view when the cavity is examined from the front. 



On the right or septal wall of the left atrium the position 

 of the margin 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 those 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, the left 

 atrio - ventricular orifice^ and a number of minute openings 

 which are the mouths of the vence cordis minima. 



The pulmonary veins convey oxygenated blood from the 

 lungs to the atrium. Through the venae minimae cordis a 

 small amount of venous blood, from the walls of the atrium, 

 is passed into its cavity. Through the left atrio-ventricular 

 orifice the blood passes from the left atrium into the left 

 ventricle. 



The openings of the pulmonary veins are situated in the 

 posterior wall of the atrium, 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 the bicuspid valve, which has been described already 

 (p. 107). The left atrio-ventricular 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- 



