CIRCULATORY APPARATUS. 83 



Cavities. It is divided by a vertical septum into two parts, 

 which are further divided into four by a transverse septum. 



The cavities above the transverse septum are called the right 

 and left auricles. 



Those below, the right and lejt ventricles. 



RIGHT AURICLE. 



On an average the walls are one-fourth of an inch thick. It 

 receives the anterior vena cava, posterior cava vena azygos, and 

 the large coronary veins, and opens into the interior of the auric- 

 ular appendix, a mass projecting from the outside of the auricle. 

 Its interior is divided by a number of transverse bands called* mus- 

 culi pectinati. 



On the posterior wall is the fossa ovalis, the remains of the open- 

 ing existing in foetal life; around its periphery is found the annulus 

 ovale or ring. 



Behind the anterior cava is an eminence, the tuberculum Loweri. 

 The anterior border of the fossa ovalis shows a fold called the 

 Eustachian valve, a remnant of foetal life. 



The auriculo-ventricular opening is found on its floor, circular 

 in outline, surrounded by a fibrous ring and closed by the tricus- 

 pid valve. 



RIGHT VENTRICLE. 



The walls are, on an average, T % of an inch thick. The apex 

 extends to only ij inches from the point of the heart. The walls 

 are uneven from the columnae carnece, or muscular columns, 

 which project into the cavity. They are of three varieties, one 

 attached by one end and the other prolonged by the chordae ten- 

 dinae, to the edges of the valves; the second, by both extremities; 

 the third, adherent through all its length. 



The tricuspid valve closes the opening into the auricle, and is 

 composed of three segments, all attached to the chorda tendina. 



The pulmonary opening is circular and leads into the pulmonary 

 artery. It is closed in by the semilunar valves, three in number, 

 which are attached also to the tendinous cords. In the centre of 

 each valve is often seen an elevation, the corpus arantii. 



LEFT AURICLE. 



Much similar in structure to that of the right side. It opens 

 into the left ventricle, and has on its superior wall the openings 



