CIRCULATION OF THE BLOOD. 105 



The Chambers of the Heart. The interior of the heart is divided 

 by a partition in such a manner as to form two chief chambers or cavities 

 right and left. Each of these chambers is again subdivided into an 

 upper and a Idwer portion, called respectively, as already incidentally men- 

 tioned, auricle and ventricle, which freely communicate one with the 

 other; the aperture of communication, however, being guarded by valves, 

 so disposed as to allow blood to pass freely from the auricle into the ven- 

 tricle, but not in the opposife direction. There are thus four cavities 

 altogether in the heart two auricles and two ventricles; the auricle and 

 ventricle of one side being quite separate from those of the other 

 (Fig. 90). 



Right Auricle. The right auricle is situated at the right part of 

 the base of the heart as viewed from the front. It is a thin walled cavity 

 of more or less quadrilateral shape prolonged at one corner into a tongue- 

 shaped portion, the right auricular appendix, which slightly overlaps the 

 exit of the great artery, the aorta, from the heart. 



The interior is smooth, being lined with the general lining of the 

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

 cavaB, 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 EustacMan 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 cor- 

 onary 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 the auricle 

 are similar elevations arranged parallel to one another, called musculi 

 pectin ali. 



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

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

 face: 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 cre- 

 scentic (Fig. 94); 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 conus 

 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 columnce 

 carnece (described at length p. 110). 



