1006 PHYSIOLOGY 



towards the base. The uppermost of these fibres form circular rings round the 

 conus arteriosus at the base of the pulmonary artery. 



The fact that the muscular fibres are continuous over both auricles 

 and over both ventricles respectively ensures the practically simul- 

 taneous contraction of each of these parts of the heart. Although 

 on coarse dissection there seems to be absolute division between 

 the muscular tissue of auricles and ventricles, it has been shown 

 by Kent, His, and others that there is continuity of muscular tissue 

 between the two parts of the heart by a special band of muscular 

 fibres, 'the bundle of His,' which rises in the wall of the right 

 auricle and passes beneath the foramen ovale and across the auriculo- 

 ventricular junction into the inter- ventricular septum. The exact 

 course of these fibres and their significance will be considered later. 

 The normal direction of the blood-flow through the heart is deter- 

 mined mainly by the valves which guard the auriculo- ventricular 

 orifices and the openings of the aorta and pulmonary artery. The auriculo- 

 ventricular valves are tubular membranes attached round the entire 

 circumference of the auriculo-ventricular ring. They are composed of 

 fibrous and elastic tissue, covered on each side with endocardium, and 

 project downwards into the cavities of the ventricles. On each 

 side the membrane is divided by deep incisions into large flaps, three 

 in number on the right side (the tricuspid valves) and two in number 

 on the left side (the mitral valves). The sail-like margins of these 

 valves are connected by thin tendinous cords to the papillary muscles, 

 which are nipple-shaped projections of the muscular walls of the 

 ventricles. By this means the edges of the valves are kept close 

 together and prevented from eversion under the strong pressure 

 exerted by the contracting "ventricle. By the downward pull of the 

 papillary muscles on the valves during the contraction of the ventricles, 

 closure is rendered more complete, the inner surface of the valves 

 being apposed over a considerable area. The action of the valves 

 is aided by the contraction of the fibres surrounding the base of the 

 heart, so that the auriculo-ventricular orifice is much smaller during 

 systole than during diastole. 



From a purely mechanical standpoint the valves guarding the 

 arterial orifices are much more perfect than those just described, which 

 depend for their efficiency on the proper contraction of the ventricular 

 wall and of the musculi papillares. Each orifice is provided with three 

 valves, each of which is semilunar in shape and attached by its convex 

 borders to the arterial wall, and presents in the middle of its free 

 border a small fibro-cartilaginous nodule, the corpus Arantii, from 

 which fine elastic fibres pass to all parts of the valve. The extreme 

 margin of the valve, the lunula, on each side of the corpus Arantii 

 is very thin, being formed of little more than the endocardium. 



