THE CIRCULATION OF THE BLOOD. 



175 



tion. There are thus four cavities in the heart the auricle and ventri- 

 cle of one side being quite separate from those of the other (fig. 147). 



Right Auricle. The right auricle is situated at the right part of the 

 base of the heart as viewed from the front. It is a thin-walled cavity 

 of more or less quadrilateral shape, prolonged at one corner into a 



Fig. 148. The left auricle and ventricle opened and a part of their anterior and left walls re- 

 moved. J^ The pulmonary artery has been divided at its commencement ; the opening into the 

 left ventricle is carried a short distance into the aorta between two of the segments of the semilunar 

 valves ; and the left part 9f the auricle with its appendix has been removed. The right auricle is 

 out of view. 1, The two right pulmonary veins cut short ; their openings are seen within the auricle; 

 1', placed within the cavity of the auricle on the left side of the septum and on the part which forms 

 the remains of the valve of the foramen ovale, of which the crescentic fold is seen toward the left 

 nand of 1' ; 2, a narrow portion of the wall of the auricle and ventricle preserved round theauriculo- 

 ventricular orifice ; 3, 3', the cut surface of the walls of the ventricle, seen to become very much 

 thinner towards 3", at the apex ; 4, a small part of the anterior wall of the left ventricle which has 

 been preserved with the principal anterior columna carnea or musculus papillaris attached to it ; 



tongue-shaped portion, the right auricular appendix, which slightly over- 

 laps the exit of the great artery, the aorta, from the heart. 



The interior is smooth, being lined with the general lining of the 



