THE CIRCULATION OF THE BLOOD. 



129 



quently used, and, indeed, until recently, it was from the study of the 

 frog's heart that the chief part of our information on the subject was 

 obtained. If removed from the body entire, the frog's heart will con- 

 tinue to beat for many hours and even days, and the beat has no appa- 

 rent difference from the beat of the heart before removal from the body; 

 it will take place without the presence of blood or other fluid within its 

 chambers. If the beats have become infrequent, an additional one may 

 be induced by mechanically stimulating the heart by means of a blunt 

 needle ; but the time before the stimulus applied produces its results 

 (the latent period) is very prolonged, and as in this way the cardiac beat 

 is like the contraction of unstriped muscle, it has been likened to a peri- 

 staltic contraction. 



There is much uncertainty about the nervous mechanism of the beat 



FIG. 119A. FIG. 119B. 



FIG. 1 19A. The Heart of a Frog CRana esculenta ' from the front. V, ventricle ; Ad, right auricle ; 

 As, left auricle; B, bulbus arteriosus dividing into right and left aortae. (Ecker. 



FIG. 119B. - The Heart of a Frog (Rana esculenta) from the back, s.v., sinus venosus opened; 

 c.s.s., left vena cava superior; c.s.d., right vena cava superior; c.t., vena cava inferior; v.p., vena 

 pulmonales; A.d., right auricle; A.S., left auricle; A .p., opening of communication between the 

 right auricle and the sinus venosus. x 2/4 3. (Ecker j 



of the frog's heart, but what has just been said shows, at any rate, two 

 things : firstly, that as the heart will beat when removed from the body 

 in a way differing not all from the normal, it must contain within itself 

 the mechanism of rhythmical contraction ; and, secondly, that as it can 

 beat without the presence of fluid with its chambers, the movement can- 

 not depend solely on reflex excitation ly the entrance of blood. 



The nervous apparatus existing in the heart itself has been found to 

 consist of collections of microscopic ganglia, and of nerve-fibres proceed- 

 ing from them. These ganglia are demonstrable as being collected 

 chiefly into three groups: one is in the wall of the sinus veuosus at the 

 9 



