358 THE HEART. 



809, 8'), the remains of the foramen ovale (vestigium foraminis ovalis), which is an 

 open passage in the foetal heart from the right to the left auricle. The fossa ovalis 

 is bounded above and at the sides by a prominent border, deficient below, the 

 annulus ovalis or isthmus Vieussenii, while the floor of the fossa, formed by what 

 was previously a valve, is thin and translucent ; and not unfrequently a small 

 oblique passage leading into the left auricle is left between it and the annular 

 border. At the hinder part of the cavity are seen the orifices of the superior and 

 inferior venae cavae, the former passing downwards and forwards, the latter, larger 

 and placed a little farther back, being directed upwards and inwards. Just below 

 the orifice of the superior cava is a slight projection, better marked in certain 

 quadrupeds than in man, which has received the somewhat misleading name of 

 tubercle of Lower. 



In front of the orifice of the inferior vena cava, and partly covering it, is a 

 crescentic fold of the lining membrane, the Eustachian valve, which is continuous 

 by its convexity with the margin of the venous orifice, while its inner cornu is pro- 

 longed into the anterior limb of the annulus ovalis. This valve, which is very 

 variable in character in the adult, being often cribriform or perforated with larger 

 holes, is an important structure in the foetal heart, and serves the purpose of direct- 

 ing the stream of blood from the inferior vena cava through the foramen ovale into 

 the left auricle. The other openings into the right auricle are 1, the auriculo- 

 ventricular aperture, situated in front of the inferior vena cava at the lower and fore 

 part of the cavity : it is oval in form and large, admitting three fingers easily ; 2, 

 the orifice of the coronary sinus of the heart (fig. 309, 3"), situated between the 

 inferior cava and the auriculo-ventricular opening : this is guarded by a semicircular 

 valve, sometimes fenestrated, which, although previously figured by Eustachius, is 

 commonly named valve of Thebesius ; 3, openings of three or four anterior cardiac 

 veins from the surface of the right ventricle ; and 4, the foramina of Thebesius, a 

 number of small pits variously situated, some of which are merely recesses closed at 

 the bottom, while others are the mouths of small veins (venw minimw cordis). 



According to L. Langer the foramina Thebesii are not confined to the right auricle but 

 occur in all the cavities of the heart ; and into some of them, even in the ventricles, small 

 veins which proceed from the muscular substance of the heart open. 



The great coronary vein (fig. 308) is considerably dilated before it enters the auricle, and 

 this dilated portion, -which is imbedded in the posterior wall of the left auricle, is termed 

 the coronary sinus. At the junction of the coronary vein with the dilated portion there is 

 a valve consisting of one or two segments. Other small veins likewise enter the coronary 

 sinus, each of them protected by a valve. One of these small veins, the oblique vein of 

 Marshall, takes a straight course from the vestigial fold before mentioned, over the back of 

 the left auricle, to open into the coronary sinus. This vein has no valve over its orifice : it, 

 together with the coronary sinus, is to be looked upon as the remnant of the original left 

 superior vena cava of the embryo (vide antea, p. 353). 



The right ventricle (fig. 306, 1) occupies the chief part of the anterior sur- 

 face of the heart, the right border, and a smaller portion of the posterior surface. 

 It extends nearly, but not quite, to the apex. Triangular in shape, its upper and 

 left angle is prolonged in a conical form to the commencement of the pulmonary 

 artery : this part of the ventricle is named conus arteriosus, or infundibulum. The 

 muscular wall of this ventricle is thickest at the base, and becomes thinner towards 

 the apex. When the cavity is laid open (fig. 309) the septum of the heart is seen 

 to bulge into it, so that in cross section this ventricle is crescentic in form (fig. 311, 

 6). At the base of the ventricle are two orifices, protected by valves ; the auriculo- 

 ventricular, of an oval form, and situated toward the right, and that of the pul- 

 monary artery, smaller, more elevated, and towards the left. Between the two the 

 wall of the cavity projects downwards, in the form of a thick rounded muscular 



