3 88 



PRINCIPLES OF GENERAL PHYSIOLOGY 



of the cer\ iml svinpathetic in another experiment. Feeding the animal, which 

 normally produces reflex stimulation of vaso-dilators to the tongue and salivary 

 glands, caused vaso-constrictor phenomena in the ear ; direct stimulation of 

 the lingual nerve produced vaso-constrictor effects instead of dilator effects. The 

 conclusion seems inevitable that the excitation process in the nerve fibre is the 

 same in all cases. 



The question has important relations to the physiology of the sense-organs 

 and is known as "Miiller's law of the specific energies of the senses." It may 



be mentioned here that 

 mechanical, chemical, or 

 electrical stimulation of 

 the chorda tympani nn\c 

 in the tympanic ca\itv 

 cause equally sensation of 

 taste. The sensation of 

 light said to be caiiM-d 

 by section of the optic 

 nerve is not quite so 

 certain, on account of the 

 possibility of disturbance 

 of the retina. 



Differences in the 

 state of excitation of a 

 single nerve fibre, accord- 

 ingly, can only be such as 

 would be brought about 

 by differences in the rate 

 at which the separate 

 stimuli follow one another. 



We see now the difficulty 

 of accepting the view of Bab- 

 kin (1913) that the different 

 effects of " secretory " and 

 " trophic " nerves to the 

 salivary glands are due to 

 qualitative differences in the 

 nature of the impulses pass- 

 ing to the gland cells along 

 fibres of the same kind, that 

 is, with similar peripheral 

 and central connections. 



That apparently quali- 

 tative differences in a 

 reflex can be obtained by 

 altering the character of 

 the stimulus is shown 

 also by the experiments 

 of Sherrington and Sow- 

 ton (1911, p. 439), where 

 the form of the reflex 

 contraction of the vasto-crureus muscle, produced by excitation of the central end 

 of the popliteal nerve of the same side, varies in form according to the time course 

 of the electrical stimulus used. For example, that produced by the rheoimi in- 

 current has the character of a long-maintained, steady contraction, without evidence 

 of fatigue and with slow subsidence after the end of stimulation; in fact, 'it has 

 the properties of the tonic contraction to be described in a later chapter (see 

 Fig. 107). Of course, there are more varied possibilities in reflexes than in the 

 phenomena to be obtained by direct excitation of the nerve observed; but it 

 must be admitted that the explanation of reflex phenomena on the hypothesis 

 of the "all or nothing" nature of the nerve impulse presents difficulties, as is 

 pointed out by Graham Brown (1913). On the other hand, the tracings of reflex 



FIG. 107. REFLEX PRODUCTION OF MUSCLE TONE BY RHEONOME 



CURRENTS AND ITS INHIBITION. VastO-CrUTCUS of de- 



cerebrate cat. 



At the top time in seconds. 



At the bottom signal of stimulation. 



Rise in the curve means contraction of the muscle. 



The first period of stimulation is that of the central end of the popliteal 

 nerve by weak rheonome currents. The form* increase*. The second 

 period of stimulation is that of the same nerve, through the same 

 electrodes, by a weak faradic current from an induction coil for one 

 second only. Inhibition immediately results. 



(Sherrington and Sowton, 1911, 1, Fig. 7.) 



