306 Royal Society. 



an arrangement naturally to be expected from an intimate union of 

 the tendinous tissue of the valve with the arterial coat. 



The structure, connexions and relations of the valves is examined 

 chiefly by means of vertical sections carried through their centres 

 and adjacent parts. Such sections of the arterial valves disclose an 

 important relation which they have with the upper border of the 

 ventricles. The aorta and pulmonary artery, expanding towards 

 their termination, are situated upon the outer edge of the ventricular 

 border before described ; the consequence of which arrangement is, 

 that the portion of valve adjacent to the vessel passes over and rests 

 upon the muscular substance, and is supported upon the inner border of 

 the free edge of the ventricles surrounding the arterial orifices. This 

 arrangement, in consequence of the small size of the parts, is not so 

 obvious at the first glance in the human heart, but is more strikingly 

 shown in an examination of the heart of any one of the larger 

 animals. This appears of importance when viewed in connexion 

 with the functions of the valves. The reflux of the blood is said to 

 be sustained by the festooned rings at the base of the valves, but in 

 fact they are thinnest at this very part, corresponding to the central 

 portion of the convexity of the valves ; and if the description pre- 

 viously given of the formation of the tendinous festooned rings be a 

 correct one, it is obvious why it is so, the thicker portions being the 

 projecting angle at the junction of two valves, to which points the 

 tendinous fibres of the valves converge. Now, inasmuch as the pos- 

 terior portion of the aortic orifice is continuous with the left auri- 

 culo-ventricular aperture, no muscular tissue of the ventricle existing 

 at this part, the posterior aortic valve, and a portion of the adjacent 

 one, have no support of this kind ; but the muscular floor of the an- 

 terior aortic valve is especially broad, and it is the corresponding 

 portion of the aorta which is particularly dilated, the posterior wall 

 descending nearly vertically. The arrangement above described ob- 

 tains in all three pulmonary valves ; but as the border as well as the 

 walls of the right ventricle are considerably thinner than those of the 

 left, the muscular floor of these valves is much narrower than in 

 the anterior aortic valve. All this is of course seen on a much 

 larger scale in the hearts of the larger animals, as the Horse and 

 Ox ; and here, where the muscular floor of the valves (more espe- 

 cially the anterior aortic) is of very considerable breadth, the ten- 

 dinous tissue of the valve may be traced over the muscular surface 

 to form the wall of the vessel. 



In the larger Ruminants there are found two considerable por- 

 tions of bone, partly surrounding the orifice of the aorta ; and smaller 

 irregular fragments are occasionally observed between the principal 

 pieces. The larger portions vary much in size and shape in different 

 hearts even of the same species. They are usually elongated and 

 curved. The chief bone, which exceeds the other considerably in 

 size, embraces the whole of the right side, and the right half of the 

 back part of the orifice of the aorta ; while the little bone, not gene- 

 rally found in the smaller Puiminants, as the Sheep, its place being 

 occupied by a portion of dense fibrous tissue, extends from the middle 



