DIFFERENCE IN ANIMALS. 669 



smaller, or less frequent, or they become smaller and less frequent simultaneously. Strong 

 stimuli cause, in addition, well-marked relaxation of the heart-muscle during diastole. 



2. In order to cause inhibition of the heart, a continuous stimulus is not necessary. A 

 rhythmically interrupted moderate stimulus suffices (v. Bezold) ; 18 to 20 stimuli per second are 

 required for mammals, and 2 to 3 per second for cold-blooded animals. 



3. Donders, with Prahl and Niiel, observed that arrest of the heart's action did not take 

 place immediately the stimulus was applied to the vagus ; but about of a second period of 

 latent stimulation elapsed before the effect was produced on the heart. 



4. If the heart be arrested by stimulation of the vagus, it can still contract, if it be excited 

 directly, e.g., by pricking it with a needle, when it executes a single contraction. [This holds 

 good only for some animals, e.g., frog, tortoise, birds and mammals. In fishes, only the 

 ventricle responds to stimulation during marked inhibition ; in the newt, only the bulbus 

 arteriosus. In the newt's heart, the sinus, auricles, and ventricle are all inexcitable to direct 

 stimulation during strong inhibition.] 



5. According to A. B. Meyer, inhibitory fibres are present only in the right vagus in the 

 turtle. It is usually stated that the right vagus is more effective than the left in other animals, 

 e.g., rabbit (Masoin) ; but this is subject to many exceptions {Landois and Langendorff). [In 

 the newt, the right vagus acts more readily on the ventricle than on the other parts of the 

 heart ; slight stimulation of the right vagus can arrest the ventricle, while the sinus and auricles 

 go on beating.] 



6. The vagus has been compressed by the finger in the neck of man (Czermalc, Concato) ; but 

 this experiment is accompanied by danger, and ought not to be undertaken. The electrotonic 

 condition of the vagus is stated in 335, III. 



7. Schiff found that stimulation of the vagus of the frog caused acceleration of the heart-beat, 

 when he displaced the blood of the heart with saline solution. If blood-serum be supplied to 

 the heart, the vagus regains its inhibitory action. 



8. Many soda salts in a proper concentration arrest the inhibitory action of the vagus, while 

 potash salts restore the inhibitory function of the vagi suspended by the soda salts. If, how- 

 ever, the soda or potash salts act too long upon the heart, they produce a condition in which, 

 after the inhibitory function of the vagi is abolished, it is not again restored. The heart's 

 action in this condition is usually arhythmical (Lowit). 



9. If the intracardial pressure be greatly increased, so as to accelerate greatly the cardiac 

 pulsations, the activity of the vagus is correspondingly diminished (J. M. Ludivig and 

 Luchsinger). 



[Differences in Animals. Perhaps the most remarkable fact in connection with the influence 

 of the vagus on the eel's heart and that of all other fishes examined, is, that vagus-stimulation 

 causes the sinus and auricle to be entirely inexcitable to direct stimulation during strong 

 inhibition. Nerve-stimulation has in this case the very peculiar effect of rendering the 

 muscular tissue temporarily incapable of responding to even the strongest direct stimuli, e.g., 

 powerful induction shocks. This would appear to be decisive evidence that the vagus acts on 

 muscle directly, and not simply on automatic motor ganglia, as was held according to the old 

 view (M f William).] 



Poisons. Muscarin stimulates the terminations of the vagus in the heart, and causes the 

 heart to stand still in diastole (Schmiedeberg and Koppe). [See p. 85 for Gaskell's views.] If 

 atropin be applied in solution to the heart, this action is set aside, and the heart begins to beat 

 again. [Atropin abolishes completely the inhibitory action of the vagus on the heart. If it 

 be injected into the jugular vein of a rabbit, the pulse-beats are increased 27 per cent., 

 in the dog, they may be trebled, and in a man under its full influence the pulse- beats 

 may rise from 70 to 150 or more. After atropin, it is impossible to arrest the action of the 

 heart by stimulation of the vagus, and in the frog this cannot be done even by stimulation of 

 the inhibitory centre in the heart itself, so that atropin must be regarded as paralysing the 

 intracardiac terminations of the vagus.] Digitalin diminishes the number of heart-beats 

 by stimulating the cardio-inhibitory centre (vagus) in the medulla. Large doses diminish the 

 excitability of the vagus centre, and increase at the same time the accelerating cardiac ganglia, 

 so that the heart-beats are thereby increased. In small doses, digitalin raises the blood- 

 pressure by stimulating the vaso-motor centre and the elements of the vascular wall (Klug). 

 Nicotin first excites the vagus, then rapidly paralyses it. Hydrocyanic acid has the same 

 effect (Preyer). Atropin (v. Bezold) and curara (large dose CI. Bernard and Kblliker) 

 paralyse the vagi, and so does a very low temperature or high fever. 



370. CENTRE FOR THE ACCELERATING CARDIAC NERVES. 



Nervus Accelerans. It is more than probable that a centre exists in the medulla 

 oblongata, which sends accelerating fibres to the heart. These fibres pass from the 

 medulla oblongata but from which part thereof has not been exactly ascertained 

 through the spinal cord, and leave the cord through the rami communicantes of 

 the lower cervical and upper six dorsal nerves {Strieker), to pass into the sympathetic 



