*o Sept., 1908.] Elements of Animal Physiology. 521 



It is still a matter of debate whether this muscular rhythm is produced 

 :by a local nervous system residing in the heart, or is to be looked on 

 as a property of the muscular substance itself. The nerves passing to 

 the heart from the central nervous system may be severed without stoppage 

 of the heart; the heart may even be cut out of the body, yet so long as 

 its vessels are supplied with blood or a saline solution of the proper 

 <:oncentration, it will beat for many hours. Even certain portions of 

 the heart, if isolated, will exhibit the same property. But the central 

 nervous system, though it is not the cause of the beat, can certainly exercise 

 some control over it. Two sets of ner\e fibres pass into the heart. In 

 the vagus nerve there are cranial autonomic fibres which can slow down 

 the rate of the beat and also diminish the force of the ventricular 

 contraction. The heart is normally under some vagus control, for if 

 these nerves be cut the beat at once increases in strength and rate. 

 Impulses can also pass down the vagus due to reflex action. Thus a 

 blow on the stomach or wind-pipe, or an irritating gas getting into the 

 upper air passages, will cause stimuli to be sent down to the heart slowing 

 and weakening it. The other set of nerves is a supply of fibres from 

 the thoracic autonomic or sympathetic system which has just the opposite 

 action on the heart, namely, quickening the beat and strengthening the 

 force. The increase in rate and strength of the heart beat during exertion, 

 •or accompanying fear, is due tO' svmpathetic impulses as well as weakening 

 of vagus control. The significance of the supply from the central nervous 

 system is that the heart-lDeat may be altered to suit the requirements of 

 the body generally. In exertion more blood is wanted in the muscles: 

 in fear the heart automatically prepares for the muscular exertion of 

 fighting or running away. 



The rate of the heart beat varies in different animals, and in each animal 

 with the age, being quicker in the young than in the adult. As averages 

 for adult animals the following may be given : — 



Horse ... ... ... 30 — 40 per minute. 



Cow ... ... ... 40 — 50 , . 



Sheep ... ... ... 60 — 80 ,, 



Man ... ... ... 70 — 80 ,, 



Dog ... ... ... 70 — 120 ,, 



In a man or a sheep, with the rate of 75 per mniute, each beat will 

 take up 0.8 of a second, of which time 0.4 is occupied with the systole 

 and the remaining half with diastole. 



At the height of systole the ventricle comes into contact with the chest 

 wall, giving rise to the so-called apex beat, which may often be 

 seen and can usually be felt at the proper region in each animal. If the 

 ear or a stethoscope te placed on the skin over the heart-region two 

 sounds can be distinctly heard, one during, and one shortly after systole. 

 The second sound is short and sharp, and is due to closure of the semilunar 

 valves ; the first sound is duller and more prolonged, and seems to be 

 made up of more than one factor ; but closure of the mitral and tricuspid 

 valves and the contraction of the muscle of the ventricle are probably the 

 chief causes. 



We are now in a position to trace the circulation in detail, and we may 

 begin with the veins entering the left auricle. These pulmonary veins 

 as thev are called, come from the lung and carry bright-red arterial blood 

 in a sluggish stream towards the auricle. The blood enters the auricle, 

 fills it out, and passes through the mitral valve into the ventricle which 

 it distends to a feeble extent, the flaps of the mitrnl valve floatin?;^ up 



