THE HEART. 



467 



and aortic orifices, the smaller to the left and behind the opening, close to the 

 wall of the ventricle. Two smaller segments are usually found at the angles of 

 junction of the larger. The mitral valve-flaps are furnished with chordae ten- 

 dineae, the mode of attachment of which is precisely similar to those on the right 

 side ; but they are thicker, stronger, and less numerous. 



The semilunar valves surround the orifice of the aorta ; two are posterior 

 (right and left) and one anterior : they are simi-lar in structure and in their mode 

 of attachment to those of the pulmonary artery. They are, however, larger, 

 thicker, and stronger than those of the right side ; the lunulae are more distinct, 

 and the corpora Arantii larger and more prominent. Opposite each segment the 

 wall of the aorta presents a slight dilatation or bulging (sinus of Valsalva). They 

 are larger than those at the commencement of the pulmonary artery. 



^Pulmonary 

 artery. 



FIG. 278. Section of the heart, showing the interventricular septum. 



The columnce carnece, admit of a subdivision into three sets, like those upon 

 the right side ; but they are smaller, more numerous, and present a dense inter- 

 lacement, especially at the apex, and upon the posterior wall. Those attached by 

 one extremity only, the musculi papillares, are two in number, being connected 

 one to the anterior, the other to the posterior wall ; they are of large size, and ter- 

 minate by free rounded extremities, from which the chordae tendineae arise. 



The septum between the two ventricles is thick, especially below (Fig. 278). 

 At its upper part it suddenly tapers off and becomes destitute of muscular fibres, 

 consisting only of fibrous tissue, covered by two layers of endocardium ; and on 

 the right side also covered, during diastole, by one of the flaps of the tricuspid 

 valve. This upper portion is termed the membranous part of the septum, and is 

 continued upward and forms the septum between the aortic vestibule and the right 



