The necessity of some means for regulating the action of the heart in accordance with the wants of the body is obvious, and in the heart we find that such an arrangement exists in relation both to the strength and rate of pulsation.

The action of the vagus upon the heart has long been a matter of great dispute, some physiologists holding it to be the motor nerve of the heart, while the majority regard it as inhibitory. The reason of this disagreement probably is that the right and left vagi have frequently different effects upon the heart, and that the effects even of the same vagus may vary according to the state of nutrition of the heart, and other circumstances. We find for example in rabbits that both the right and left vagi can usually slow or stop the heart; but sometimes the right has much greater power in this respect than the left, and in some species of tortoise the left vagus has no inhibitory action upon the heart at all, and in the frog during the breeding season the action of the vagi is very uncertain. The cause of these different results appears to be that the vagus is a very complex nerve, and contains accelerating and strengthening fibres which are derived from the sympathetic, as well as inhibitory fibres which are derived from the spinal accessory, and sensory fibres which belong to the vagus proper. The results of stimulating the vagus trunk will vary according to the proportion of these different fibres which it contains, and on the activity of each kind at the time of stimulation.

A number of experiments made by Gaskell on the heart in situ and with the clamping apparatus already mentioned, by which the beats of the auricle and ventricle may be simultaneously recorded, have led him to divide the effects produced on the heart by irritation of the vagi into two types: (a) affections of the rate of rhythm; and (b) affections of the strength of the contractions.

The effect of vagus stimulation on the heart of the frog may be divided into five classes.

The 1st class is that which occurs with the heart of the tortoise or frog in situ or just after removal from the body. The vagus here causes arrest by slowing the rate of rhythm; and, in consequence, the first beats which occur after the heart again begins to beat are slower than those preceding the stimulation.

In the next classes the vagus produces its effect by weakening the strength of the contractions so that they may become invisible and the heart remains still, but after it begins to beat their rate is as quick or quicker than before.

Fig. 112.   After Gaskell. Tracing showing the action of the vagus on the heart. Aur. indicates the auricular, and Vent. the ventricular tracing. The part included between the upright lines indicates the time during which the vagus was stimulated.

Fig. 112. - After Gaskell. Tracing showing the action of the vagus on the heart. Aur. indicates the auricular, and Vent. the ventricular tracing. The part included between the upright lines indicates the time during which the vagus was stimulated. ft 8 indicates that the secondary coil used for stimulation was eight centimetres distant from the primary. The part of the tracing to the left hand shows the regular contractions of moderate height before stimulation. During stimulation, and for some time after, the movements of both auricle and ventricle are entirely arrested. After they again commence they are small at first, but soon acquire a much greater amplitude than before the application of the stimulus.

The 2nd class is an example of this. In it irritation of the nerve produces complete stoppage of both auricles and ventricles. This is followed by contractions, which are at first so small as to be hardly visible, but quickly grow larger until they are much greater than the normal; from this they gradually decrease to the normal size (Fig. 112).

The two types of action may occur together, the rhythm becoming slower and the contractions smaller. This is seen in Fig. 113.

Fig 113

Fig. 113

After Gaskell. Tracing showing diminished amplitude and slowing of the pulsations without complete stoppage, during irritation of the vagus.

The 3rd class is where irritation produces no still-stand of either auricles or ventricles, but only great diminution in the size of the beats, followed by a gradual increase and subsequent fall similar to that just described. This curve is like the first, but differs from it in the absence of the complete arrest (Fig. 114).

Regulating Action of the Nervous System 159

Fig. 114. - After Gaskell. Tracing showing diminished amplitude of contraction without slowing or stoppage during irritation of vagus.

The 4th is that where there is no primary diminution, but gradual increase in the size of the beats, which again sink to the normal (Fig. 115).

The 5th is where irritation of the vagus does not stop the beats of the venous sinus but causes both auricles and ventricle to stop.

The ordinary inhibitory effect of the vagus is the one which is noticed best in well-nourished hearts, and as the heart becomes more exhausted, and is dying, the motor power of the vagus becomes more and more pronounced. We find a similar occurrence in the case of the splanchnics, which lose their inhibitory power as the intestine dies. Nervous structures as a rule die sooner than muscle, and the conclusion is not unwarranted that the disappearance of the inhibitory action of the vagus is due to a gradual death of the nervous structures upon which it acts in the healthy heart, while its action on the muscular tissue, which has a more prolonged vitality, still remains. The actual increase, indeed, in its motor action we may attribute to the removal of nervous interference.