EXCITATION AND INHIBITION 419 



more about the former also. It is to be feared that the consideration to be given 

 to certain theories of inhibition will leave us with very little of definite value. It 

 is necessary to do this, however, since some theories, proposed as of more or less 

 general application, will be found to be inapplicable to any case whatever. It 

 appears, on superficial examination, that we must consider the possibility of two 

 different modes of inhibition. When a smooth muscle is in a state of tonus, 

 due to its own inherent properties, the action of an inhibitory nerve is to quell 

 this state of excitation by acting on the excitable substance of the muscle directly. 

 When the state of excitation is due to the action of an excitatory nerve fibre upon 

 the muscle cell, it would seem more appropriate to block the point of synapse where 

 the nerve fibre conveys the excitation to the muscle. Similarly, a nerve cell may 



FIG. 126. TONUS OF RESPIRATORY CENTRE AND EFFECT OF THE VAGUS NERVE THEREON. 

 Tracing of diaphragm slip of rabbit by Head's method (see page 418). 



Movement downwards indicates contraction of the diaphragm. 



At the beginning, with the vagus nerves intact, the slip was completely relaxed, showing very small respiratory 

 contractions. The series of large contractions was produced by the addition of carbon dioxide to the air 

 breathed. 



At x , the vagi were cut. It will be noted that the diaphragm passed into partial tonus. 



A second inhalation of air containing carbon dioxide caused increased respiratory movements, and it is interesting 

 to note that, between each respiration, the tone was relaxed. 



Time marker in seconds at the bottom. 



(Obtained by Prof. Starling in a demonstration.) 



be in a state of excitation, due to chemical changes in itself or to chemical 

 influence of constituents of the blood, and, in this case, inhibition may be brought 

 about by action on the actual cell substance. If, on the contrary, the state of 

 excitation is due to nerve impulses acting through synapses with the cell, it would 

 seem to be a simpler hypothesis to suppose that the action of an inhibitory nerve 

 fibre is exercised on the synapse, making it impermeable to the excitatory impulses. 

 The cell substance itself is thus left free to respond to other stimuli. It does 

 not seem impossible that experiment may decide whether there is any real 

 distinction of this kind, and it is quite likely that it is the synapse that is 

 concerned in all cases. Sherrington (1906, p. 103), in fact, is inclined to regard 

 the process of inhibition as of the same fundamental nature in all cases, whether 

 on nerve cell or on peripheral muscle. 



Physical Interference. Although it is possible that this view is not seriously 

 advocated at the present time by any competent physiologist, there are still 

 traces of it to be found, especially in certain work of the Verworn school. We 



