THE CIRCULATION 215 



and is continuous with, the right posterior wall of the aorta, 

 and hangs down into the ventricle between the aortic and 

 auriculo-ventricular orifices, thus dividing the ventricle into 

 two parts, an aortic and an auricular part. This cusp is 

 very strong, and in many animals bone is developed in it 

 towards its base. It is composed of dense fibrous tissue, is 

 smooth on both sides, and the chordae are inserted chiefly 

 along its edges. 



2nd. The posterior or left cusp takes origin from the 

 back part of the auriculo-ventricular ring, and hangs in the 

 ventricle in its relaxed state against the posterior and left 

 wall. It is smaller and less strongly made than the anterior 

 cusp. The chordae tendineae are not only inserted into its 

 edge, but run up along its posterior aspect to be inserted 

 into the auriculo-ventricular ring, and they thus give the 

 posterior aspect of the cusp a rough ridged appearance. 



The chordae from the anterior papillary muscle are inserted 

 into the anterior or left edge of both cusps, those from the 

 posterior papillary muscle into the posterior or right edges of 

 both cusps.- When the papillary muscles contract, the cusps 

 are thus drawn together. The edge of each cusp thins out 

 to form a delicate border, which, when the cusps are approxi- 

 mated, completely seals the aperture. 



On the right side of the heart the auriculo-ventricular 

 orifice is separate from the pulmonary opening, and the three 

 cusps of the tricuspid valve are developed in connection 

 with the crescentic opening from the auricle (Fig. 106). One 

 rises from the ring above the septum, and hangs down into 

 the ventricle upon the surface of the septum. This cusp is 

 small, thin, and delicate. It is attached by its lower border 

 to the septal papillary muscles. The chief or infundibular 

 cusp (Fig. 107, I.C.} rises from the front part of the ring 

 between the pulmonary infundibulum and the auriculo- 

 ventricular opening. It is connected by its anterior border 

 with the horizontal fibres from the superior papillary muscles, 

 and by its lower and inferior border by the chordae from the 

 anterior papillary muscle. When these two sets of papillary 

 muscles contract, this cusp is drawn flat against the bulging 

 septum. 



The posterior cusp (P. C.) takes origin from the posterior 



