46 CIRCULATION OF THE BLOOD 



especially, it may be stated that the muscular fibres of the auricles and 

 of the upper part of the ventricles have the property of intermittent and 

 regular contraction, which is dependent, to a great extent, on the influ- 

 ence of the so-called motor ganglia of the heart ; and that the wave of 

 contraction is transmitted to the lower portion of the ventricles, the fibres 

 of which do not seem to possess the property of independent contraction. 

 The muscular tissue of the heart, however, may be thrown into contrac- 

 tion during diastole by the application of a stimulus, a property observed 

 in all muscular fibres. The excitability manifested in this way is more 

 marked in the interior than on the exterior of the organ. Blood in con- 

 tact with the lining membrane of the heart acts as a stimulus in a re- 

 markable degree and is even capable of restoring excitability after it has 

 become extinct. The passage of blood through the heart is a natural 

 stimulus and is an important element in the production of regular pul- 

 sations, although it does not endow the fibres with their contractile 

 properties. 1 



Accelerator Nerves. Experiments on the influence of the sympa- 

 thetic nerves on the heart have not been entirely satisfactory. It has been 

 observed that the action of the heart is arrested by destroying the car- 

 diac plexus ; but in regard to this, the difficulty of the operation and the 

 disturbance of the heart consequent on the necessary manipulations 

 must be taken into account. It has been shown, however, that stimula- 

 tion of the sympathetic in the neck has the effect of accelerating the car- 

 diac movements. 



In the bulb is a centre, stimulation of which increases the rapidity 

 of the heart's action ; and from this centre, fibres descend in the substance 

 of the spinal cord, pass out with the communicating branches of the 

 lower cervical and upper dorsal nerves to the sympathetic and go to the 

 cardiac plexus. In the cat, the accelerator fibres pass through the first 

 thoracic sympathetic ganglion. Taking all precautions to eliminate the 

 influence of variations in blood-pressure, it has been shown that after 

 division of the pneumogastrics, stimulation of the accelerator fibres in- 

 creases the number of beats of the heart. This action is direct and not 

 reflex. 



1 The well-known experiments of Stannius, published in 1852, have lately received much 

 attention. In these observations it was found that a ligature applied exactly at the line of junc- 

 tion of the sinus venosus with the right auricle, in the frog, arrests for a time the contractions 

 of other parts of the heart, while the sinus continues to beat regularly ; but the walls of the other 

 cavities will contract under direct stimulation. The wave of contraction se'ems to pass from the 

 sinus to the auricle and ventricle. If, now, another ligature is applied to the line of junction of 

 the auricle with the ventricle, the ventricle resumes its rhythmical contractions, while the auricle 

 continues at rest. Explanations of these phenomena which are not observed in animals higher 

 in the scale are so unsatisfactory that it does not seem worth while to discuss them in the body 

 of the text. 



