54 CIRCULATION OF THE BLOOD ACTION OF THE HEART. 



plain its rapid and feeble action in certain cases of anaemia. The heart, 

 therefore, although capable of independent action, is excited to contraction 

 by the blood as it passes through its cavities. A glance at the succession of 

 its movements, particularly in cold-blooded animals in which they are so 

 slow that the phenomena can be easily observed will show how these con- 

 tractions are produced. There is first a distention of the auricle, and this is 

 immediately followed by a contraction filling the ventricle, which in its 

 turn contracts. Undoubtedly, the tension of the fibres, as well as the con- 

 tact of blood in its interior, acts as a stimulus ; and as all the fibres of each 

 cavity are put on the stretch at the same instant, they contract simul- 

 taneously. The successive and regular distention of each cavity thus produces 

 rhythmical and forcible contractions ; and the mere fact that the action of 

 the heart alternately empties and dilates its cavities insures regular pulsa- 

 tions, so long as blood is supplied and no disturbing influences are in 

 operation. 



The intermittent contraction and successive action of the fibres of the 

 heart, when the organ has been removed from the body, are dependent, to a 

 great extent, upon sympathetic ganglia situated near its base. If the 

 ventricle of a frog's heart be divided transversely at the upper third, the 

 lower two-thirds will no longer contract spontaneously, while the auricles 

 and the upper third of the ventricle continue to pulsate. If a stimulus be 

 then applied to the lower two-thirds of the ventricle, this is usually followed 

 by a single contraction, and not by a series of more or less regular pulsations. 

 It has been observed, also, that small, detached pieces of the auricles will 

 pulsate regularly for a time. 



In the frog there are three ganglia closely connected with the heart ; one 

 at an expansion of the inferior vena cava just before it enters the auricle, 

 called the venous sinus (Remak), another between the left auricle and the 

 ventricle (Bidder), and a third between the two auricles (Ludwig). Accord- 

 ing to Robert Meade Smith, the first two ganglia communicate the motor 

 impulse to the muscular fibres of the heart. The third is the inhibitory 

 ganglion, and this regulates, through its action upon the motor ganglia, the 

 transmission of motor impulses. " As regards the manner in which these 

 ganglia produce the rhythmical contraction of the heart, little is known ; 

 but that they are the prime factors in producing not only the rhythm of 

 the cardiac revolutions, with its various modifications, but also the starting 

 point of each individual contraction, is one of the best established facts in 

 physiology." 



In man and in most warm-blooded animals, collections of sympathetic 

 ganglia are found attached to the nerves at the line of junction of the 

 auricles with the ventricles. 



Nearly all of the experiments just referred to were made upon the hearts 

 of cold-blooded animals, particularly the frog; but in all animals, under 

 normal conditions, the contractions of the heart seem to start from the 

 auricles. The fact, however, that the ventricles will contract regularly in a 

 living animal, after the excitability of the auricles has been exhausted by 



