36 A MANUAL OF VETERINARY PHYSIOLOGY 



and though the internal diameter is being reduced in every 

 direction, this does not disturb the accuracy with which the 

 valves apply themselves to each other. The papillary muscles 

 compensate for the approximation of the ventricular walls by 

 constantly shortening, and through the chordce tendinecB main- 

 tain the segments of the valves in apposition and prevent further 

 encroachment on the auricle. 



The semilunar or sigmoid valves, which guard the entrance of 

 the aorta and pulmonary artery, are composed of fibrous tissue, 

 and possess at the centre of each segment a small hard body, the 

 corf us Arantii, which is particularly well marked in the aortic 

 valves. It is generally supposed that these shot-like bodies 

 complete the central sealing when the valves are closed, but this 

 view causes too little attention to be paid to the fact that the 



valves not only meet at their 

 free border, but overlap. Chau- 

 veau, with his finger in the 

 pulmonary artery of the horse, 

 states that he has tried to hold 

 back one flap in order to render 

 the opening patent, but the 

 two remaining segments applied 

 themselves so closely to his finger 

 that the orifice was closed. That 



Fig. 16.— Tricuspid Valve of the ,, bodies are an additional 



Horse in Closed Position seen | n f se D °aies are an aaaitionai 



from the Auricle. help to the completely tight clos- 



Note the cracks in the surface, which mg of the valves IS undoubted, 

 represent where the margin of the but the overlapping of the valves 

 valves meet and fold in against fe ^ mQst important facto r. 

 each other like the lips of a tooth- f 



less mouth. When the sigmoid valves are 



not in action, they still lie in the 

 blood-stream, and not against the wall of the vessels, as was at 

 one time supposed, nor do those in the aorta cover the opening of 

 the coronary arteries. It is probable that the valves are enabled 

 to stand out in the blood-stream through the action of vortex 

 currents, and while thus waiting for their turn in the heart's 

 cycle they form a triangular orifice with curved sides. 



It is generally believed that both the aortic and pulmonary 

 valves are closed by the regurgitation of the blood ; but it has 

 been pointed out that as the blood is leaving both ventricles, it is 

 streaming through orifices which at that time are mere chinks, 

 owing to the pads of heart muscle which take their origin from 

 all sides of the mouth of the vessels. Vortices are thus created 

 in the space between the arterial root and the edge of the valves. 

 These vortices tend to press the edges of the valves together, 

 and the valves consequently close the moment the blood actually 



