176 HANDBOOK OF PHYSIOLOGY. 



heart, the endocardium, and into it open the superior and inferior venae 

 cavse,'or great veins, which convey the blood from all parts of the body 

 to the heart. The former is directed downward and forward, the latter 

 upward and inward; between the entrances of these vessels is a slight 

 tubercle called tubercle of Lower. The opening of the inferior cava is 

 protected and partly covered by a membrane called the Eustachian 

 valve. In the posterior wall of the auricle is a slight depression called 

 the fossa ovalis, which corresponds to an opening between the right and 

 left auricles which exists in foetal life. The right auricular appendix is 

 of oval form, and admits three fingers. Various veins, including the 

 coronary sinus, or the dilated portion of the right coronary vein, open 

 into this chamber. In the appendix are closely set elevations of the 

 muscular tissue covered with endocardium, and on the anterior wall of 



Fig. 149. Transverse section of bullock's heart in a state of cadaveric rigidity. (Dalton.) 

 b. Cavity of right ventricle, a. Cavity of left ventricle. 



the auricle are similar elevations arranged parallel to one another, called 

 mu sculi pectinati. 



Rigid Ventricle. The right ventricle occupies the chief part of the 

 anterior surface of the heart, as well as a small part of the posterior 

 surface : it forms the right margin of the heart. It takes no part in 

 the formation of the apex. On section its cavity, in consequence of the 

 encroachment upon it of the septum ventriculorum, is semilunar or 

 crescentic (fig. 149); into it are two openings, the auriculo-ventricular 

 at the base and the opening of the pulmonary artery also at the base, 

 but more to the left; the part of the ventricle leading to it is called the 

 conns arteriosus or infundibulum ; both orifices are guarded by valves, 

 the former called tricuspid and the latter semilunar or sigmoid. In 

 this ventricle are also the projections of the muscular tissue called co- 

 lumns car new (described at length p. 179). 



Left Auricle. The left auricle is situated at the left and posterior 

 part of the base of the heart, and is best seen from behind. It is quad- 

 rilateral, and receives on either side two pulmonary veins. The auricu- 

 lar appendix is the only part of the auricle seen from tlie front, and 

 corresponds with that on the right side, but is thicker, and the interior 

 is more smooth. The left auricle is only slightly thicker than the right. 

 The left auriculo-ventricular orifice is oval, and a little smaller than 



