232 VETERINARY PHYSIOLOGY 



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 tendinese 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. 



When the papillary muscles contract, the cusps are drawn 

 together. The edge of each cusp thins out to form a delicate 

 border, which, when the cusps are approximated, 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 trieuspid valve are developed in connection with the 

 crescentic opening from the auricle (fig. 108). One rises from 

 the ring above the septum, and hangs down into the ventricle 

 upon the surface of the septum. This cusp is small, triin, and 

 delicate. It is attached by its lower border to the septal 

 papillary muscles. The chief or infundibular cusp (fig. 109, 

 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 with 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.O.) takes origin from the posterior and 

 outer part of the ring, and hangs down into the posterior part 

 of the ventricle. It is connected by its anterior margin with 

 the anterior papillary muscle and by its posterior margin with 

 the posterior papillary muscle. Contraction of these muscles 

 therefore approximate its anterior edge to the infundibular 

 cusp, its posterior edge to the septal cusp, and pull it towards 

 the septum. 



In both the infundibular and posterior cusps many of the 

 chordae pass up to be inserted into the auriculo-ventricular ring. 



Semilunar Valves. The valves, situated at the opening of the 

 ventricles into the great arteries, are also formed as special 

 developments of the endocardium. 



Each is composed of three half-moon-shaped membranous 



