The Heart. 53 



traction of the heart, but the length of the pause (B. 

 Sanderson). 



While these changes are occurring in the cavities of the 

 heart, the organ itself undergoes some distinct changes in 

 shape and movement as the result of them. During 

 systole the heart becomes narrower from side to side, but 

 increased in width from front to rear ; it is reduced in all 

 its diameters excepting its vertical one, and the reduction 

 of its horizontal diameter assists materially in closing the 

 auricular ventricular valves. It also alters in shape ; during 

 the period of diastole it is an oblique cone with an elliptical 

 base ; during systole it is an upright cone with a rounded 

 base. This change in shape is accompanied by a rotation 

 of the heart to the right, and a tilting of the apex to the 

 left (Hermann). 



This tilting outwards of the heart, due to the spiral 

 arrangement of its fibres, may assist in producing the impulse 

 against the chest wall, which in the horse occurs between 

 the fifth and sixth ribs. Colin insists that in the horse the 

 impulse is not given by the apex alone, but by the lower 

 half of the wall of the left ventricle. Experiments made on 

 some animals prove that the apex does not leave the chest 

 wall, but that the impulse is due to the hardening of the 

 ventricular wall in contact with the chest wall. This 

 impulse against the chest agrees with the closure of the 

 auriculo-ventricular valves, and the bursting open of the 

 sigmoid valves (Sanderson). In the horse the apex does not 

 rest on the ribs, but is free and directed towards the sternum. 



The action of the valves of the heart during a cardiac 

 cycle is peculiar and interesting. We have described 

 how the auriculo-ventricular curtains are floated up as 

 the ventricle fills, and how with increased pressure they 

 come together, being prevented by the chordae tendinse 

 from being carried too far ; we have also noticed how the 

 reduction in the horizontal diameter of the heart assists 

 the action of these valves ; further, experimental inquiry 

 has determined that when the ventricular systole is at its 

 height these valves bulge upwards into the auricles, 



