CIRCULATION OF THE BLOOD. 



125 



the heart by means of a blunt needle; but the time before the stimulus 

 applied produces its result (the latent period) is very prolonged, and as 

 in this way the cardiac beat is like the contraction of unstriped muscle, 

 the method has been likened to a peristaltic contraction. 



There is much uncertainty about the nervous mechanism of the beat 

 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 at 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 within its chambers, the movement cannot 

 depend merely on reflex excitation by the entrance of blood. The nervous 

 apparatus existing in the heart itself consists of collections of microscopic 

 ganglia, and of nerve- fibres proceeding from them. These ganglia are 



-AA AA 



FIG. 106. Heart of frog. (Burdon-Sanderson after Fritsche.) Front view to the left, back view 

 to the right. A A. Aortae. V. cs. Venae cavae superiores. At s, left auricle. At d, right auricle. 

 Fen., ventricle. B. ar., Bulbus arteriosus. S. v., Sinus venosus. V. c. i., Vena cava inferior. V. 

 ft ., Venae hepaticae . V . p . , Venae pulmonales . 



demonstrable as being collected chiefly into three groups; one is in the 

 wall of the sinus venosus (Remakes); a second, near the junction between 

 the auricle and ventricle (Bidder's); and the third in the septum between 

 the auricles. 



Some very important experiments seem to identify the rhythmical 

 contractions of the frog's heart with these ganglia. If the heart be re- 

 moved entire from the body, the sequence of the contraction of its several 

 beats will take place with rhythmical regularity, viz., of the sinus veno- 

 sus, the auricles, the ventricle, and bulbus arteriosus, in order. If the 

 heart be removed at the junction of the sinus and auricle, the former will 

 continue to beat, but the removed portion will for a short variable time 

 stop beating, and then resume its beats, but with a rhythm different to 

 that of the sinus: and, further, if the ventricle be removed, it will take 

 a still longer time before recommencing its pulsation after its removal 

 than the larger portion consisting of the auricles and ventricle, and its 

 rhythm is different from that of the unremoved portion, and not so regu- 

 lar, nor will it continue to pulsate so long: during the period of stop- 

 page a contraction will occur if the ventricle be mechanically or otherwise 

 stimulated. If the lower two-thirds or apex of the ventricle be removed, 

 the remainder of the heart will go on beating regularly in the body, but 



