CIRCULATION OF THE BLOOD. 



107 



Fro. 94. Transverse section of bullock's 

 heart in a state of cadaveric rigidity, a, 

 cavity of left ventricle. 6, cavity of right- 

 ventricle. (Dalton.) 



There is a slight vestige of the foramen between the auricles, which 

 exists in fatal life, on the septum between them. 



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 corre- 

 sponding 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 toward the right. In this 

 ventricle, as in the right, are the co- 

 lumnae carneae, which are smaller but 

 more closely reticulated. They are 

 chiefly found near the apex and along 

 the posterior Avail. 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 exception 

 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 auricles 

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

 considerably less. The latter condition 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 blood before the auricles contract. 



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

 long, 3 inches greatest width, and 2|- 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 diminishes 

 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 

 ventricle, and forms the boundary of the auriculo-ventricular opening. 

 Fibrous tissue also exists at the origins of the pulmonary artery and aorta. 



The muscular fibres of each auricle are in part continuous with those 

 of the other, and partly separate; and the same remark holds true for the 

 ventricles. The fibres of the auricles are, however, quite separate from 

 those of the ventricles, the bond of connection between them being only 

 the fibrous tissue of the auriculo-ventricular openings. 



The muscular fibres of the heart, unlike those of most of the involun- 



