THE THORA>t '^7*1 



held close together and side by side, can be passed through it with- 

 out difficulty. Through this opening the blood passes from the 

 right auricle into the right ventricle, its return being prevented by 

 the tricuspid valve, whi^h will be described in connection with the 

 right ventricle. 



The so-called hiberde of Lower is an unimportant slight projec- 

 tion, which is situated on the interauricular septum, just below the 

 opening of the superior vena cava and behind the upper part of 

 the fossa ovalis. 



The posterior wall of the atrimn corresponds to the interauricular 

 septum, and it presents for consideration the fossa ovalis and the 

 annulus ovalis. 



The fossa ovalis is an oval depression which is situated upon the 

 lower part of the interauricular septum, a little above and to the 

 left of the orifice of the inferior vena cava. It indicates the position 

 of the foramen ovale of the foetal heart, which is a conmiunication 

 between the t\vo auricles through which the blood entering the 

 right auricle by the inferior vena cava passes into the left auricle. 

 The floor of the fossa ovalis is very thin, and is bounded above and 

 at the sides by a prominent, crescentic margin, called the annulus 

 ovalis. The annulus is deficient below, and the concavity of the 

 crescent is directed downwards. Its anterior limb is continuous 

 with the vestigial Eustachian valve. In some cases a minute 

 oblique communication between the two auricles persists in the 

 adult, being situated imder cover of the upper portion of the 

 annulus ovalis. 



Interior of the Right Ventricle. — WTien exposed to view in the 

 ordinary way the interior of the right ventricle is triangular, the 

 base being directed backwards and upwards, and the apex for- 

 wards and downwards. It is completely separated from the left 

 ventricle by the inter\'entricular septum, which forms the posterior 

 wall of the ventricle, and bulges into it so as to be convex towards 

 it. In transverse section, therefore, the right ventricle is semilunar. 

 Its wall, which is about three times thinner than that of the 

 left ventricle, is thickest at the base, and becomes thinner towards 

 the apex. The capacity of the ventricle is about 4 ounces. Its 

 upper and left part forms the infimdihuluni or coitus arteriosus, 

 from the upper part of which the trunk of the pulmonary 

 artery' springs. The walls of the infundibulum are smooth, but 

 elsewhere the walls of the ventricle are elevated into muscular 

 bands, called columnce carnece. These project into the cavity, and, 

 from their reticular arrangement, they render the wall verj- irregular. 

 According to the manner in which the columnae comeae are attached 

 to the wall, they are arranged in three sets : (i) some are simple 

 elevations, which are attached to the wall by their entire length, 

 as well as by their extremities ; (2) some are attached to the wail 

 only by their extremities, being free elsewhere ; and (3) others are 

 attached only by one extremity. These latter are called musculi 

 fapillarcs. They are conical, and their bases are attached to the 



62 



