100 HANDBOOK OF PHYSIOLOGY. 



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 

 carnew (described at length p. 103). 



(3.) Left Auricle. The left auricle is situated at the left and poste- 

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

 quadrilateral, and receives on either side two pulmonary veins. The 

 auricular appendix is the only part of the auricle seen from the front, 

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

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

 right, the difference being as 1^- lines to 1 line. The left auriculo-ven- 

 tricular orifice is oval, and a little smaller than that on the right side of 

 the heart. There is a slight vestige of the foramen between the auri- 

 cles, which exists in fcetal life, on the septum between them. 



(4.) Left Ventricle. Though taking part to a comparatively slight 

 extent in the anterior surface, the left ventricle occupies the chief part 

 of the posterior surface. In it are two openings very close together, 

 viz., the auriculo-ventricular and the aortic, guarded by the valves cor- 

 responding to those of the right side of the heart, viz., the bicuspid or 

 mitral and the semilunar or sigmoid. The first opening is at the left 

 and back part of the base of the ventricle, and the aortic in front and 

 towards the right. In this ventricle, as in the right, are the columns 

 carneae, which are smaller but more closely reticulated. They are chiefly 

 found near the apex and along the posterior wall. They will be again 

 referred to in the 'description of the valves. The walls of the left ven- 

 tricle, which are nearly half an inch in thickness, are, with the excep- 

 tion of the apex, twice or three times as thick as those of the right. 



Capacity of the Chambers. The capacity of the two ventricles is 

 about four to six ounces of blood, the whole of which is impelled into 

 their respective arteries at each contraction. The capacity of the auri- 

 cles is rather less than that of the ventricles : the thickness of their 

 walls is considerably less. The latter is adapted to the small amount of 

 force which the auricles require in order to empty themselves into their 

 adjoining ventricles ; the former to the circumstance of the ventricles 

 being partly filled with the blood before the auricles contract. 



Size and Weight of the Heart. The heart is about 5 inches long, 3 

 inches greatest width, and 2J inches in its extreme thickness. The 

 average weight of the heart in the adult is from 9 to 10 ounces ; its 

 weight gradually increasing throughout life till middle age ; it dimin- 

 ishes in old age. 



Structure. The walls of the heart are constructed almost entirely of 

 layers of muscular fibres ; but a ring of connective tissue, to which some 

 of the muscular fibres are attached, is inserted between each auricle and 



