56 A MANUAL OF VETERINARY PHYSIOLOGY 



into it through sensory nerves ; a mechanism so arranged pro- 

 duces what is known as a reflex tonus. 



The Action of Poisons on the Heart has been appealed to in order 

 to decide not only their pharmacological effect, but also as a 

 physiological means of research in connection with its nervous 

 mechanism. Atropine causes the heart-rate to become quick- 

 ened, due to the inhibitory effect of the vagus being suspended. 

 If the vagus under these conditions be stimulated, no inhibition 

 follows, and it is argued that this is due to paralysis of the nerve 

 terminations of the inhibitory fibres in the heart muscle, much 

 as the motor end-plates are in muscles under the action of 

 curare. Muscarine, a poison obtained from certain mushrooms, 

 causes the heart to slow down, and finally to stop. Pilocarpine 

 has much the same effect, and it is assumed that both these 

 alkaloids stimulate the inhibitory fibres. This view is strength- 

 ened by the fact that atropine abolishes the inhibitory effect 

 produced by muscarine and pilocarpine, and it is considered that 

 the result is due to the paralysis of the terminals of the inhibitory 

 fibres in the heart muscle. 



Function of the Sympathetic Nerve. — The course of the accelera- 

 tor fibres derived from the sympathetic has been previously 

 described. Their distribution in the heart is mainly to the 

 ventricles. The vagus fibres, it will be remembered, are princi- 

 pally distributed to the auricles. If the sympathetic trunk in 

 the neck of the mammal be stimulated, no effect follows, for, as 

 Fig. 21 shows, there are no accelerator fibres above the inferior 

 cervical ganglion. If certain branches issuing from this ganglion 

 be stimulated, the heart-beats are increased in frequency, and 

 sometimes, but not always, in force ; in other cases they are 

 increased in force and not in frequency. There is reason to 

 think these differences are explained by the probability that the 

 accelerator fibres consist of two sets, one increasing the fre- 

 quency of the heart, and the other increasing the force. From 

 either the right or left ganglion accelerator effects can be 

 obtained on stimulation, but the augmentor effect is best obtained 

 from the left side. 



The accelerator fibres, like the inhibitory, are in a state of con- 

 stant or tonic activity, as evidenced by the fact that, if they be 

 divided, the heart-rate is thereby decreased. It has been assumed 

 that this constant activity is carried out by a centre situated in the 

 medulla, and that to it afferent impulses pass of a reflex nature 

 which either increase or decrease its activity. No better example 

 of a reflex stimulation of the accelerator centre could be witnessed 

 than is shown in the fright of a startled nervous horse ; the heart 

 may almost be heard thumping against the chest wall. 



No such accelerator centre has actually been located in the 



