406 



PRINCIPLES OF GENERAL PHYSIOLOGY 



supplied with two sets of nerves, exciting and inhibiting. The natural state of 

 the muscle appears to be one of tonic contraction, which can be reduced by certain 

 nerve fibres. It then remains in this state until a stimulation of the exciting nerve 

 fibres sets it into contraction again. Further details with regard to these tonus 

 phenomena will be found in Chapter XVIII. It is important to note that, as 

 Pavlov points out (pp. 28 and 29), there are no nerve cells in the muscle itself, so 

 that the two kinds of influence must be exercised on the muscle cells directly. In 

 view of later theories of receptive substances, it may be added that Pavlov states 

 that there must be special parts in the interior of the muscle fibres on which the 

 contraction depends, and others on which the relaxation depends. 



The Heart. In Fig. 113 tracings are given of the stoppage of the heart of the 

 tortoise by stimulation of the vagus nerve. It will be noted that, in addition to 

 cessation of the spontaneous beats, the state of tone in diastole is reduced. In 

 Fig. 114 a curve of the action of the opposite kind of nerves, the accelerator or 

 augmentor nerves, from the sympathetic system, is shown. More detailed analysis 

 of the nervous regulation of the heart will be found in Chapter XXIII. 



It was remarked above/ that it is to be supposed that the phenomena asso- 



FIG. 114. ACTION OF THE SYMPATHETIC NERVE (AUGMENTOR) ON THE HEART OF THE TOAD. 

 Suspension method. Clamp in auriculo- ventricular groove. Sympathetic stimulated 

 before it joins the vagus. 



(Gaskell, 1900, Fig. 111.) 



ciated with inhibition will naturally be of an opposite kind to those associated 

 with excitation. In this connection, the observations made by Gaskell (1887) 

 are of much interest. We have seen that the wave of excitation in muscle 

 is accompanied by electrical negativity of the spot actually in the state of 

 excitation. Now Gaskell showed how a preparation can be made of the auricle 

 of the tortoise in such a way that spontaneous beats cease for a time, owing to 

 separation from the place where these beats arise in the sinus, while the supply of 

 inhibitory fibres is left intact in a nerve (the coronary), which is separate from the 

 other tissues, and can be left when they are cut. We have also seen that if one 

 electrode is situated on an injured spot, we obtain only the electrical change taking 

 place at the other electrode on a normal spot. When the auricle contracts, then, 

 there is a movement of the galvanometer needle in a direction indicating negativity 

 of the auricular muscle under an electrode on a normal spot, when the other 

 electrode is on an injured spot. Now, suppose that we excite the vagus nerve 

 while the auricle is in a state of rest after separation from the sinus. Gaskell 

 saw that the muscle becomes electro-positive ; the galvanometer needle moved in 

 the opposite direction to that associated with contraction (Fig. 115). It might, 

 perhaps, be supposed that the explanation of this change is merely that the 

 auricle, although showing no rhythmic beats, was in a state of slight tonic con- 

 traction, which would, of course, be associated with a permanent negative deflec- 

 tion of the galvanometer. If the vagus caused a disappearance of this tonus, the 



