312 OF THE CIRCULATION OF THE BLOOD. 



arrest occurred. But in the case of ordinary muscles the period of latent 

 excitation, or the time that elapses between the application of a stimulus to 

 the nerve and the beginning of the contraction of the muscle, does not ex- 

 ceed youth of a second. Secondly, Meyer 1 has been able by direct irritation 

 of a particular part of the sinus venosus in the common eel by means of an 

 interrupted current to arrest the heart's action for sixteen minutes, and in 

 the frog he was occasionally able by persistent direct irritation to keep the 

 heart at rest for an hour. M. Foster 2 has in like manner pointed out that 

 in certain Molluscs, Anodon and Helix, there is no nerve corresponding to 

 the Vagus, but that the heart can be stopped in diastole by direct irritation 

 when removed from the body. The inhibitory mechanism must in these in- 

 stances be in the heart itself. Lastly, evidence of an intermediate apparatus 

 such as is represented in the above figure as the iutracardiac inhibitory 

 centre is derived from the action of certain poisons. 3 Thus Nicotin and 

 Woorara when injected subcutaueously in very small doses (ith to M of a 

 milligramme) appear to stimulate the inhibitory centres, and the heart beats 

 slower ; but they soon paralyze these centres, and the heart then beats faster, 

 and when this occurs irritation of the Vagus will no longer inhibit the 

 heart's action, but if whilst the heart is beating faster Muscarin be admin- 

 istered or the sinus venosus be irritated the heart still stops in diastole. If, 

 however, Atropin have been injected, not only does the heart beat faster, but 

 it can no longer be stopped in diastole by irritation of the sinus veuosus, or 

 the administration of Muscariu. The explanation of this appears to be that 

 whilst Nicotin and Woorara in certain doses and stages of their action para- 

 lyze the vagal centre, and the Cardio-iuhibitory fibres of the Vagus, so that 

 these can no longer transmit inhibitory impulses, they do not paralyze the 

 Intracardiac inhibitory centres, as is indicated by the effect of direct irrita- 

 tion on the sinus veuosus. Atropin, on the other hand, not only paralyzes 

 the Vagal centre, but it also paralyzes the intracardiac inhibitory centres, 

 for the heart under its influence is no longer capable of being arrested in 

 diastole, either by irritation of the -vagus or of the sinus venosus. 4 Dr. 

 Bruuton is disposed to think that Atropin acts especially on the connecting 

 filaments, c F, between the inhibitory and cardio-motor centres. 5 In the 

 course of an elaborate series of experiments upon the action of the nerves 

 on the heart, v. Bezold observed that in animals under the influence of 

 Woorara, in which therefore all muscular movements that might complicate 

 the phenomena were prevented, on applying electrical excitation to the upper 

 cut surface of the divided Vagus, the blood-pressure fell, and he also ob- 

 served that similar irritation applied to the medulla oblongata caused con- 

 siderable elevation of blood-pressure, which still occurred, even when the 

 two Vagi had been divided. He concluded that the spinal cord contained 

 fibres acting directly on the heart. He further noticed that on division of 

 the spinal cord in the cervical region the blood-pressure immediately fell, 

 whilst if the posterior cut surface of the cord was irritated the blood-pres- 

 sure again rose. The conclusion drawn by v. Bezold was, that the medulla 



seem to show that if the inhibitory influence is very strongly exerted it prevents the 

 production of motor energy in the motor ganglia, whilst if it be moderuti'ly excited 

 it docs not interfere with its generation, but only with its discharge. 



1 Das Hemmungsnerven-system, IHii'.t. 



2 I'lliigor's Archiv f. Phys.. Bd. v, p. 191. 



3 See tlie papers of Trnhart, Inaug. Dissert., Beitriig. zur Nicotin- Wirkung, Dor- 

 pat, 18(39; and Sclimii-deberg, Ludwig's Arboiten, Bd. v, 1871, p. 41. 



4 The effect, of the administration of Atropin in man is to raise the number of the 

 pulse from 72-80 to 140-180 in the minute. 



s Brit. Med. Journ., Dec. 10th, .1871, p. 689. 



