90 THE FROG CHAP, vi 



Circulation of the Blood. We see then that an inves- 

 tigation of the structure of the heart shows that fluid can 

 traverse it in one direction only, viz., from the sinus to 

 the right auricle, from the auricles to the ventricle, from 

 the ventricle to the conus, and from the conus to the 

 bulb us aortae, and so to the arteries. The valves in the 

 veins are so 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 re- 

 turned 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 to- 

 gether 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 (systole) 

 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 (diastole). 



The course of the blood through the heart will now 

 be clear. When the sinus (Fig. 23, 5. v) contracts, the 



