546 PRINCIPLES OF GENERAL PHYSIOLOGY 



regarded, no doubt correctly, as being for the most part due to increased blood 

 supply from the rise of arterial pressure caused by the adrenaline. At the same 

 time, the investigators are inclined to think that there is also a direct effect, since 

 that on the muscle lasts longer than the rise of blood pressure. But might it not 

 be the result of the previous increase of blood supply? More convincing is the 

 experiment in which the blood pressure in the limb under experiment was 

 prevented from rising by compression of the artery. The effect was still present. 

 According to Panella (1907), adrenaline is an antagonist to curare, hence it must 

 act on some receptive substance in the muscle. This is confirmed by Gruber (1914). 

 It is clear that some of the remarkable characters of the tonic state of skeletal 

 muscle may- be explained by the existence of the sympathetic innervation, but 

 further evidence is required. Recently, Kuno (1915) has failed to obtain any 

 evidence of an action of adrenaline on the voluntary muscle of the frog. Nor has 

 he been able to confirm the production of tonus tlrrough the sympathetic rami. 



TONUS IN NERVE CENTRES 



The constant activity of certain nerve centres, such as the respiratory and 

 vasomotor centres, is well known. 



The question has been argued as to whether the state of excitation in such 

 cases is actually automatic, or whether it is kept up by afferent impulses from the 

 periphery. There can be no doubt that such impulses are able to modify the state 

 of the centres, and also that chemical substances, such as hydrogen ions, present 

 in the blood, are able to set up a state of excitation in nerve centres. We have 

 seen (page 457) that the heat regulating centre is accessible to the direct action 

 of heat and cold. 



On the whole, then, we must admit that afferent nerve impulses are not 

 always necessary. It is naturally a question not easy of experimental attack 

 to decide whether there is an automatic state of excitation apart from stimu- 

 lating substances in the blood, although it does not seem impossible. There are, 

 obviously, many centres which must not be active, except for special purposes ; 

 such are those of the voluntary muscles. Even here, however, the question arises 

 as to whether the inactivity of centres not in use may not be due to inhibition 

 (see the experiments of Pavlov on conditioned reflexes, pages 503-506). 



The tracing given in Fig. 126 (page 418) is of interest in the present connection. 

 The fact shown there suggests that the respiratory centre is naturally in a state 

 of tonic excitation ; since, when the afferent impulses from the lungs are cut off 

 by section of the vagus nerves, the diaphragm passes into a state of partial tonic 

 contraction. 



SUMMARY 



Involuntary, smooth muscle, in its various situations, is capable of remaining 

 in a state of moderate contraction, or tonus, independent of any influence from 

 nervous centres. 



Certain muscular systems in invertebrates, such as that which closes the 

 shell in bivalve molluscs, and many others, are able to maintain a shortened state 

 against a heavy load for a long time without any evidence of fatigue. 



This state of " fixation " may occur at any length of the muscle fibres, and 

 with any tension. 



A similar condition is met with in the urinary bladder of mammals, and 

 probably in the muscular coat of the arterioles, as well as in other situations. 



It may be compared to the putting into action of a " catch " or ratchet 

 mechanism. 



To put the " catch " into action, or to remove it, requires the intervention of 

 nerve impulses from centres ; these impulses would be excitatory in the first case, 

 inhibitory 'in the second. They are conveyed by distinct nerve tracts. 



