38 A MANUAL OF VETERINARY PHYSIOLOGY 



blood, and a wave of contraction which first appears at the 

 vessels leading into them passes over the chambers ; the auricular 

 appendage becomes pale, the auriculo - ventricular groove is 

 drawn upwards, and the auricles, by a sudden sharp and brief 

 contraction, empty their contents into the ventricle. At this 

 moment a backward positive wave is produced in the anterior 

 vena cava, which shows itself by a pulsation in the jugular veins 

 at the root of the neck, well seen in the horse. 



The effect of the auricular contraction is to complete the filling 

 of the ventricles and close the valves by raising the pressure in 

 those cavities. During the whole time the valves are open the 

 ventricle is filling ; what the systole of the auricle effects is the 

 final filling of the ventricle. The auriculo-ventricular valves, 

 which during the filling have been gradually passing into position, 

 are now, under the sudden increase of intraventricular pressure, 

 rapidly closed, and bulge into the auricle, their closure producing 

 the first sound of the heart. The valves are prevented from 

 going too far by the chorda tendinece, which are acted upon by the 

 gradually contracting musculi fapillares, as previously explained. 

 At this moment the blood imprisoned in the ventricles is shut 

 off from the auricles by the closed auriculo-ventricular valves, 

 and shut off from the aorta and pulmonary artery by the closed 

 semilunar valves. The ventricles now contract and raise the 

 pressure within their cavities in order to force open the semilunar 

 valves, for until the pressure within the ventricle exceeds that 

 in the aorta and pulmonary artery, these two avenues of escape 

 are closed. While the intraventricular pressure is being increased 

 by the walls contracting tighter and tighter on the imprisoned 

 blood, the heart is changing in shape ; it is becoming more 

 globular, its walls are growing tenser, it is shortening from base 

 to apex, and in its writhing, screwing efforts to overcome the 

 pressure in the aorta and pulmonary artery it is twisting slightly 

 from left to right, and from before backwards, rotating on its 

 vertical axis, and so brings more of the left ventricle against the 

 sides of the chest. The intraventricular pressure is now sufficient 

 to cause the aortic and pulmonary valves to yield, and blood 

 rushes into these vessels under the systole of the ventricles. The 

 aorta and pulmonary artery fill with blood, elongate and curve ; 

 the heart rotating so long as the ventricles are contracting makes 

 its impulse against the chest wall, and empties its contents into 

 the arteries. The auriculo-ventricular groove now moves down- 

 ward towards the apex of the heart, the ventricular walls relax 

 and elongate, the pressure within them falls, and the blood from 

 the over-full arteries is prevented from regurgitating into the 

 ventricles by the semilunar valves coming into position, and the 

 closure of these valves creates the second sound of the heart. 



