DISSECTION OF THE iSACK AND THOEAX. 129 



towards the cavity. It will be observed that its wall is about thrice 

 the thickness of that of the right cavity, a circumstance which makes 

 It easy to distmguish the right and left ventricles in the undissected 

 heart ; for, whereas the former appears flabby, the latter is firm and 

 solid-looking. The left ventricle possesses columnce carnece like those on 

 the right side. The musculi papillares are two in number, and are of 

 very large size. They are placed on the wall, and are provided with 

 chord<s tmdinew stronger than those of the right cavity. The base of 

 the cavity shows the left aurimlo-ventricular opening, which is somewhat 

 smaller than the right. It is guarded by a valve with two large cusps, 

 and hence called the bicuspid valve. It is also very commonly 

 designated the mitral valve, from a fancied resemblance to a bishop's 

 mitre. The cusps are stronger than those of the tricuspid valve, with 

 which they agree in shape and disposition. Two smaller segments 



Fig. 11. 

 Root of the common Aorta laid open. 



1, 1, 1. Semilunar segments of the aortic valve ; 2. Corpus Arantii ; 3, 3. Orifices of right and left 

 coronary arteries from two of the sinuses of Valsalva ; 4. Ventricular wall ; 5. Arterial wall. 



alternate with the main ones. In mode of action the mitral exactly 

 resembles the tricuspid valve. When the ventricle contracts, the blood, 

 prevented from regurgitating into the auricle, is forced out of the 

 cavity along the great systemic artery — the common aorta, which springs 

 from the right-anterior part of the base of the ventricle. The aortic 

 orifice is guarded by a three-segmented semilunar valve. These segments 

 are stronger than those at the mouth of the pulmonary artery, which 

 they otherwise exactly resemble. Opposite to each a large sinus of 

 Valsalva is developed on the wall of the artery, and from two of these 

 spring the right and left coronary arteries of the heart. 



STRUCTURE OF THE HEART. 



^^In structure the heart consists of a muscular wall, an external serous 

 investment — the epicardium, and an internal serous lining — the endocar- 

 dium. The valves are folds of the endocardium, strengthened with fibrous 



