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 left 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 veifts, and opens into the interior of the 

 auricular appefidix, a mass projecting from the outside of the 

 auricle. Its interior is divided by a number of transverse bands 

 called musculi pectinati. 



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

 opening existing in foetal life : around its periphery is found the 

 aniiulus ovale or ring. 



Behind the anterior cava is an eminence, the tubeixuhmi 

 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, y^^ of an inch thick. The apex 

 extends to only \\ 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 chordcB tendincB. 



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 



