308 THE HEART. 



catures of the endocardium, and of enclosed fibrous structure, which varies 

 in thickness at different parts. A tendinous band strengthens the free 

 margin of the valve, and is attached at the middle of that margin to a slight 

 fibro-cartilaginous thickening, the nodulus or corpus Arantii. Other ten- 

 dinous fibres, spreading out from the attached border of the valve, run 

 into the valve and towards the nodule, occupying its whole extent, except 

 two narrow lunated portions, one on each side, adjoining the free margin of 

 the valve. These parts, which are named lunulce, are therefore thinner than 

 the rest. There is also a strengthening fibrous cord surrounding the 

 attached border of each valve. (For further information regarding the 

 structure of the valves, consult Pettigrew, in Trans. Roy. Soc. Edin. 

 1864.) 



During the contraction of the ventricle the valves lie against the sides of 

 the artery, and allow the blood to flow freely past them ; but during the 

 ventricular diastole, when the column of fluid in the artery is partially 

 thrown back by the elasticity of the coats of that vessel, the sigmoid valves 

 are distended by the regurgitating blood, and completely close the arterial 

 orifice. When the valves are thus closed, the whole free border and the 

 thin lunated parts are closely applied to each other, and are held together 

 as well as exempted from strain by the opposite and equal pressure of the 

 blood on either side, so that the greater the pressure the more accurate 

 must be the closure. The force of the reflux is sustained by the stouter 

 and more tendinous part of the valve. (Retzius, in Miiller's Archiv., 1843, 

 p. 14.) 



Opposite the pouches formed by the sigmoid valves, the commencing 

 aorta and pulmonary artery present dilatations or recesses in their walls, 

 called sinuses of Valsalva, which give to the transverse section of the vessel 

 a trilobate form ; and, as one of these is placed behind each segment of the 

 valve, it thus forms along with it a cup-shaped cavity. 



SPECIAL DESCRIPTION. The detailed anatomy of the four cavities of the 

 heart may be conveniently considered in the order of the course of the 

 blood in its double circulation through them from the vense cavse to the 

 aorta. 



1. The RIGHT AURICLE forms the right anterior and lower part of the 

 base of the heart, and is in contact below with the pericardium where it 

 lies upon the diaphragm : it receives blood in two large streams, from 

 the superior and the inferior vense cavse, besides the smaller quantity from 

 the coronary vein. At its fore part its auricular appendage projects for- 

 wards and to the left, in front of the aorta, as far as the pulmonary artery. 

 This part is triangular in form, compressed and slightly dentated at its 

 border, and has more strongly muscular walls than the sinus venosus. 

 The auricular appendage and anterior wall of the sinus venosus are 

 closely ridged in a vertical direction with musculi pectinati ; the remaining 

 parts of the walls are comparatively smooth. The superior vena cava is 

 directed downwards and forwards into the upper part of the auricle, while 

 the inferior vena cava terminates in the lower and back part by a consider- 

 ably larger opening, which is directed upwards and inwards. In the floor 

 of the auricle, in front of the inferior vena cava, is the auriculo-ven- 

 tricular opening, leading into the right ventricle ; it is oval in form, and 

 about an inch and a quarter in diameter, admitting three fingers easily. 

 The other foramina opening into the right auricle are, the orifice of the large 

 coronary vein of the heart, situated between the opening of the inferior 

 vena cava and the auriculo- ventricular opening, and a number of small pits, 



