2i 4 HUMAN PHYSIOLOGY 



papillary muscles just under the pulmonary orifice, and 

 having a horizontal direction, their apices pointing back- 

 wards the superior papillary muscle (Fig. 107, S.P.M.). 



2nd. A large papillary muscle taking origin from the mass 

 of fleshy columns at the apex of the ventricle. It is directed 

 upwards. It may be called the anterior papillary muscle 

 (AJP.M.). 



3rd. One or more papillary muscles of varying size arising 

 from the posterior part of the apical portion of the ventricle 

 and constituting the posterior papillary muscle (P. P.M.). 



4th. A number of small septal papillary muscles arising 

 from the septum. 



The distribution of the chorda? from these muscles will be 

 considered in connection with the auriculo-ventricular valves. 



In contraction, the outmost and inmost layers of the 

 ventricles tend to approximate the apex to the base of the 

 ventricles, but this is resisted by the contracting middle layer. 

 The apex tends to be tilted towards the right, the papillary 

 muscles shorten, the colummu cameo; by their shortening and 

 thickening encroach upon the ventricular cavity, and help 

 to abolish it, while the auriculo-ventricular rings are drawn 

 downwards and inwards towards the septum. 



The endocardium forms a continuous fibrous foyer, lined 

 by endothelium, extending from the vessels over the inner 

 aspect of auricles and ventricles. At certain points flaps 

 of this endocardium are developed to form the valves of 

 the heart. 



In the heart, valves are situated at the entrance to and at 

 the exit from the expelling cavities. There is thus on each 

 ' side of the heart a valve between the auricles and ventricles, 

 \ and a valve between the ventricles and the great arteries. 



Auriculo-ventricular Valves, On each side of the heart 

 the auriculo-ventricular valve is formed by flaps of endocar- 

 dium, which hang downwards from the auriculo-ventricular 

 ring like a funnel into the ventricular cavity, and are attached 

 to the apices of the papillary muscles by the chordae tendinese 

 (Figs. 107 and 108). 



On the left side, of the heart there are two main cusps, 

 forming the mitral valves (Fig. 108) 



1st. An anterior or right cusp, which takes origin from, 



