60 PHYSIOLOGY OF FARM ANIMALS [CH. 



These are of the nature of pouches, three in number, and having 

 their free edges inside pointing away from the ventricle. These 

 free edges separate under pressure and admit of the passage of 

 the blood from the ventricle, but close together when the pressure 

 is in the opposite direction, the pouches being then distended. 

 The name semilunar valves has been given to them owing to 

 their shape. 



The blood is kept in motion mainly by the rhythmical con- 

 traction and dilatation of the heart, the alternating periods or 

 conditions being called systole and diastole. Systole begins with 

 the great venous trunks (the two superior venae cavae and 

 the inferior vena cava) and then passes to the right auricle which 

 undergoes a sharp contraction. The left auricle into which the 

 pulmonary veins open contracts simultaneously with the right 

 auricle. The wave of contraction then extends through the 

 tissue dividing the auricular from the ventricular part of the 

 heart to the ventricles which contract very powerfully, forcing 

 the blood into the pulmonary arteries on the right side and the 

 aorta on the left. The heart then expands previous to another 

 contraction. As is well known the movements of the heart are 

 entirely involuntary. Nevertheless the organ is well provided 

 with nerves, there being two chief sources of supply, those from 

 the vagus or tenth cranial nerve, and those arising from the 

 sympathetics. The former are inhibitory, checking or controlling 

 the heart's activity, while the sympathetic nerves are accelerator 

 in function. There is a cardio-inhibitory centre in the hind brain, 

 and when this is stimulated to excess (as in fainting or sudden 

 emotion) it temporarily (or sometimes permanently) stops the 

 heart's action. On the other hand stimulation of the sympathetic 

 supply quickens the heart beats and may induce palpitation. 

 Apart however from the nerve supply it is the essential property 

 of heart muscle to undergo rhythmical contractions as may be 

 seen when the organ is withdrawn from the body and placed in 

 a glass vessel, while at the same time it is perfused with a fluid 

 resembling blood serum (e.g. Ringer's solution). Under these 

 conditions the heart will continue to beat in the same manner as 

 it does normally when fulfilling its usual bodily functions. More- 

 over, in the case of some animals (e.g. tortoises) even small pieces 

 of heart muscle will continue to undergo contractions after being 

 removed from the rest of the heart. 



