90 THE FROG CHAP, yi 



arranged as to allow the blood in these vessels to flow only, 

 towards the heart. The experiment of cutting the vessels 

 shows that the blood in the arteries does actually flow from 

 the heart, that in the veins towards the heart. We thus 

 demonstrate that there is not merely a movement but a true 

 circulation of the blood, the current starting from the heart, 

 passing by the arteries to all parts of the body, and being 

 returned to the heart by the veins. 



Action of the Heart. The circulation of the blood is 

 effected by the pulsation of the heart. This organ is 

 made of muscle ; each of its cavities is to be considered 

 as a bag, the walls of which are formed of muscular fibres 

 crossing one another in various directions and encircling 

 the cavity. We have seen that when an ordinary spindle- 

 shaped muscle contracts, its two ends are brought nearer 

 together. When a hollow muscular bag contracts the effect 

 will be to squeeze the walls together and so diminish the 

 cavity. Hence when any chamber of the heart contracts 

 it must expel a part or the whole of its contained blood. 

 The contraction of the chambers of the heart takes place in 

 regular order : first the sinus, then the two auricles together, 

 then the ventricle, and lastly the conus. The contraction 

 in each case is visible as a sort of throb and is followed 

 by a period of rest, during which the chamber regains its 

 former dimensions. 



The course of the blood through the heart will now be 

 clear. When the sinus (Fig. 23, s.v) contracts, the con- 

 tained blood, which, coming by the precavals and postcaval, 

 is non-aerated, is acted upon in all directions and might 

 therefore be forced either into the three great veins (pr.cv.v, 

 pt.cv.v) or into the right auricle (r.au). But the veins are 

 full of blood steadily flowing towards the heart, and any 

 regurgitation is further prevented by their valves : the right 



