CAUSE OF THE CONTRACTIONS OF THE HEART. 



55 



repeated stimulations and they have ceased to pulsate, shows that the so- 

 called pulsating wave coming from the auricles is not absolutely essential to 

 the contraction of the ventricles. 



Finally, in view especially of the results of experiments upon the cold- 

 blooded animals, it may be stated that the muscular fibres of the auricles 

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

 and regular contraction, which is dependent, to a great extent, upon the 

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

 contraction is transmitted to the lower two-thirds 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 contraction 

 during diastole by the application of a stimulus, a property which is observed 

 in all musular fibres. The excitability manifested in this way is much more 

 marked in the interior than on the exterior of the organ. Blood in contact 

 with the lining membrane of the heart acts as a stimulus in a remarkable 

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

 extinct. The passage of blood through the heart is the natural stimulus of 

 the organ and is an important element in the 

 production of regular pulsations, although it by 

 no means endows the fibres with their contractile 

 properties. 



Accelerator Nerves. Experiments on the in- 

 fluence of the sympathetic nerves upon the heart 

 have not been entirely satisfactory. It has been 

 observed that the action of the heart is immedi- 

 ately arrested by destroying the cardiac plexus; 

 but with regard to this, the difficulty of making 

 the operation and the disturbance of the heart 

 consequent upon the necessary manipulations 

 must be taken into account. It has been shown, 

 however, that stimulation of the sympathetic in 

 the neck has the effect of accelerating the car- 

 diac movements. 



According to Strieker, there exists in the me- 

 dulla oblongata a centre, stimulation of which in- 

 creases 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 



gf variations in the blood pressure, it has been shown that after division of 

 the pneumogastric, stimulation of the accelerator fibres increases the number 

 of beats of the heart. This action is direct and not reflex. 



VAC. 



FIG. 22. Scheme of the course of 

 the accelerans fibres (Stirling). 



p, pons ; MO, medulla oblonga- 

 ta ; v, inhibitory centre for 

 the heart ; A, accelerans cen- 

 tre ; VAG., vagus ; SL, supe- 

 rior, IL, inferior laryngeal ; 

 sc, superior cardiac ; H, 

 heart ; c, cerebral impulse ; 

 s. cervical sympathetic; a, a, 

 accelerans fibres. 



