THE CIRCULATION OF THE BLOOD. 99 



the ventricles both front and back, and separates the one from the other. 

 The anterior groove is nearer the left margin and the posterior nearer 

 the right, as the front surface of the heart is made up chiefly of the 

 right ventricle and the posterior surface of the left ventricle. In the 

 furrows run the coronary vessels, which supply the tissue of the heart 

 with blood, as well as nerves and lymphatics imbedded in more or less 

 fatty material. 



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 lower portion, called respectively, as already incidentally 

 mentioned, auricle and ventricle, which freely communicate one with 

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

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

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

 altogether in the heart the auricle and ventricle of one side being quite 

 separate from those of the other (Fig. 89). 



(1.) 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 venae 

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

 to the heart. The former is directed downwards and forwards, the latter 

 upwards and inwards; 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 the auricle are similar 

 elevations arranged parallel to one another, called musculi pectinati. 



(2.) Right Ventricle. The right ventricle occupies the chief part 

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

 rior 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, in semilunar or 

 crescentic (Fig. 91) ; into it are two openings, the auriculo-ventricular 

 at the base, and the opening of the pulmonary artery alsp_at the.base, , 



