636 SPECIAL PHYSIOLOGY. 



in thickness as life advances ; but the right remains unaltered after the 

 period of full development. 



At the left and hinder part of the base of this ventricle, is the oval 

 opening from the left auricle ; in front and to the right of this, is the 

 circular aperture of the aorta. These openings are, after death, 

 smaller than the corresponding orifices on the right side of the heart. 

 The annexed Table shows the circumference of all four apertures, in 

 the adult male and female (Peacock) : 



MALE. FEMALE. 



Inches. Lines. Inches. Line. . 



Auriculo-ventricular openings, \ j^i. ' * ' " * o 7 o -in 



Arterial openings,. ' . >' 4 f 



The left auriculo-ventricular opening corresponds to the centre of 

 the sternum, reaching upwards a little to the left. It is guarded by 

 a valve, resembling the tricuspid valve, but formed of two segments 

 instead of three, and hence called the bicuspid or mitral valve (Fig. 

 106, 6). The two segments are named, from their relative position, 

 anterior and posterior ; the former is somewhat the larger. The seg- 

 ments are provided with chordce tendinece, fewer in number than those 

 of the tricuspid valve, but having similar attachments ; all these struc- 

 tures are stronger and thicker than those of the right ventricle. The 

 internal surface of the left ventricle generally, like that of the right, 

 is provided with three kinds of columnce carnece, which, however, are 

 relatively small and numerous ; there are only two musculi papillares. 



The round orifice of the aorta lies behind the junction of the third 

 left costal cartilage with the sternum. It is separated from the au- 

 riculo-ventricular opening by the base of the anterior segment of the 

 bicuspid valve, here joined to the aortic fibrous ring. The aortic ori- 

 fice is protected by three semilunar valves (Fig. 106, 8, Fig. 107, &, 

 2), resembling those of the right side in form, in their mode of attach- 

 ment to the sides of the great bloodvessel, and in the peculiar direction 

 of their free edges towards the artery ; but they are thicker and 

 stronger, have their corpora Arantii larger, and their lunular margins 

 more developed. The pouches, or sinuses of Valsalva, at the base of 

 the aorta, are also larger than those of the pulmonary arteries. The 

 two coronary arteries, or nutrient arteries of the heart, arise from the 

 bottom of two of these pouches, close behind the corresponding semi- 

 lunar valves. 



The cavities of the heart are lined by a very fine serous membrane, 

 named the endocardium, which is continuous with the lining membrane 

 of the large vessels ; it is somewhat thicker in the auricles than in the 

 ventricles, and thicker in the left than in the right cavities ; the valves 

 of the heart consist essentially of folds of this membrane, inclosing 

 fibrous tissue. It is difficult to determine the capacity of the cavities 

 of a muscular organ like the heart ; the estimates given of the capacity 

 of the left ventricle, vary from 4 to 6.3 oz. ; the right ventricular cav- 

 ity is usually said to be a little larger. The capacity of each auricle 

 corresponds with, or is a little smaller than, that of the respective 

 ventricle. 



