310 THE HEAET. 



margin of the inferior vena cava. This valve, which in the foetus is pro- 

 portionally large, and serves, in conjunction with the ammlus ovalis, to 

 direct the blood from the inferior vena cava through the foramen ovale, is in 

 the adult comparatively small, and very variably developed, being often cribri- 

 form or perforated by numerous foramina, and sometimes reduced to a few- 

 slender filaments, or even altogether wanting. The mouth of the coronary 

 vein is likewise protected by a semicircular valve, which is sometimes 

 double, and which, though previously figured by Eustachius, is often named 

 the valve of Thebesius. The coronary vein is considerably dilated before it 

 enters the auricle, and this dilated portion, which has muscular parietes, 

 is commonly termed the 4< coronary sinus." At the junction of the coronary 

 vein with this 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. 



The superior and inferior venae cavse being both directed somewhat 

 towards the left side at their terminations, the wall of the auricle presents 

 internally a convexity between them, which has received the somewhat 

 misleading name of tubercle, of Lower. In the human subject this elevation 

 is slight, but in certain quadrupeds it is more strongly marked. 



Kunning upwards from the fossa ovalis, under cover of the annulus, there some- 

 times exists a small slit, which leads beneath the annulus into the left auricle, 

 forming thus an oblique and valved aperture between the two cavities. More rarely 

 the foramen ovale of the foetus remains so patent after birth as to interfere with the 

 proper course of the circulation, and produce the pathological condition known as 

 cyanosis by the mixture of some dark or venous blood with the bright red or 

 arterial blood of the left auricle. 



2. The RIGHT or ANTERIOR VENTRICLE extends from the right auricle 

 towards the apex, and from the upper and anterior part of its base sends 

 upwards, in front and to the left of the auriculo-ventricular opening, a 

 smooth conical prolongation, free from columiiae carnese, and named infundi- 

 bulum or conus arteriosus : from the extremity of this prolongation of the 

 ventricular cavity the pulmonary artery arises. The superficial wall of this 

 ventricle, which is much thicker than that of the auricle, but thinner than 

 that of the left ventricle, is formed by the part of the heart situated to 

 the right of the anterior longitudinal groove, viz., the right border, the 

 larger part of the anterior surface, and a part of the posterior. The internal 

 or left wall is formed by the septum ventriculorum, and is convex, bulging 

 to the right into the ventricle, so that a transverse section of the cavity 

 presents a crescentic figure. 



The valve guarding the right auriculo-ventricular opening is named the 

 tricuspid valve, from the number of its segments. One of the segments, 

 the smallest, is placed towards the left and rests upon the septum of the 

 ventricles ; the other two are placed more to the right, one posteriorly 

 against the right wall of the cavity, while the other, the largest of all, lying 

 anteriorly, is interposed between the auricular and arterial openings, and 

 has its ventricular surface directed forwards and upwards. The chordae 

 teiidiiiese arise chiefly from the musculi papillares, but some also from the 

 walls of the ventricle and especially from the septum. Those arising from 

 one papillary muscle or group of muscles run up in the angular interval 

 between two adjacent segments of the valve, and, diverging from each other, 

 are attached to both segments. 



The semilunar valves at the root of the pulmonary artery are more 



