IRRITABILITY OF' THE AUTOMATIC MOTOR CENTERS. 117 



current. In the latter event the ventricular beat sometimes precedes that of the 

 auricles. If, now, the auriculo-ventricular junction be divided, the ventricle 

 begins to pulsate, partly because the procedure stimulates Bidder's ganglion, and 

 partly because the heart is no longer under the influence of the vagus, which had 

 been stimulated by the first division. If the division at the auriculo-ventricular 

 junction is made in such a way as to leave Bidder's ganglion in the auricle, the 

 latter would pulsate and the ventricle remain at rest; if the ganglion is divided 

 into two halves, both the auricles and the ventricles pulsate, because each is 

 stimulated by its own half of the ganglion. 



2. When the ventricle alone is divided in the frog's heart by ligature or in- 

 cision at the auriculo-ventricular furrow, the sinus and the auricles continue to 

 beat undisturbed, while the ventricle is arrested in diastole; the ventricle responds 

 to a local irritant with a single contraction. If the incision is made in such a way 

 as to leave the lower edge of the interauricular septum attached to the ventricle, 

 the latter also continues to pulsate. In the case of the rabbit's heart, also, the 

 ventricles continue to pulsate if a small strip of the auricles is preserved, separated 

 from the auricular nerves. 



3. Experiments performed by A. Fick in 1874 first showed that the irritative 

 process in the contractile tissue of the frog's heart is propagated in all directions 

 and that the entire frog's heart acts in a measure like a single continuous muscle- 

 fiber. Thus, for example, a transverse incision, involving the ventricle of the 

 frog's heart, does not prevent the appended flap from taking part in the systolic 

 contraction. This is shown also by the following experiments of Engelmann. If 

 the heart is cut into strips, as by zigzag incisions, in such a manner that the 

 individual pieces remain in connection with one another by means of muscle- 

 substance, the strips pulsate in regular succession, in whatever way they may be 

 connected with one another, as a result of the direction of the incisions. The 

 velocity of propagation, under such circumstances, is from ten to thirty millimeters 

 in the second. These experiments also confirm the observation that the continuous 

 stimulus that propagates the contraction is not conducted by nerve-paths but 

 by the substance of the contractile mass. 



4. When the apex of the heart has been separated from the rest of the organ 

 by a ligature it ceases to take part in the contraction of the heart, which continues 

 to pulsate; a direct stimulus, such as a stab of the apex, is followed' by only a 

 single contraction. If the heart is filled with saline solution under pressure (both 

 of which act as stimuli) , the apex will continue to pulsate. The same thing is 

 observed after poisoning with delphinin or quinin. If a cannula is tied in the 

 ventricle from a point above the auriculo-ventricular junction to the apex, the 

 latter is likewise arrested; if, however, the apical portion is filled through this 

 cannula with oxygenated blood under steady pressure, the apex will pulsate. 



The excised apex of the heart resting spontaneously, when stimulated by 

 induction-currents, responds to the weakest efficient stimulation by a maximal 

 contraction; but the application of tetanizing currents is not followed by true 

 tetanus. Closing and opening the constant current applied to the severed apex 

 give rise only to the ordinary closing and opening contractions. 



5. When the point of ligation is within the auricles, the pulsations of the 

 heart occur in successive periods (group -f ormation) , and the contractions often 

 increase in strength by regular gradations (stair-case ascent) . 



6. When the bulb of the aorta, which is devoid of ganglia, is isolated by con- 

 striction (frog), it continues to pulsate when the internal pressure is moderate; 

 after it has ceased beating, a single stimulus will give rise to a series of renewed 

 contractions. The number of contractions is increased by raising the temperature 

 to 35 C. and by increasing the internal pressure. 



7. The isolated venae cavae and their sinuses exhibit normal contractions. If 

 they are still connected with the heart they will control the movements of the 

 heart, that is, contraction of the entire heart may be induced from the position 

 of each of the large veins and the rhythm of the heart may be thus influenced. 

 Conduction takes place only through the muscle-substance and not through the 

 nerves. Porter maintains with regard to the hearts of the dog and the cat that 

 any part of the heart that is excised may continue to pulsate if only it be suffi- 

 ciently nourished. 



In opposition to the doctrine that has just been expounded, namely 

 that the stimulating influence is sent out by the cardiac ganglia, it may 

 be observed that this theory has recently begun to waver. In view of the 



