474 PRINCIPLES OF GENERAL PHYSlOLOGV 



nerve fibres to degenerate. The vaso-dilatation produced by mustard depends 011 

 the integrity of sensory nerve fibres, although not an ordinary reflex. There 

 seems to be no other explanation except that given by Bruce himself, namely, that 

 the phenomenon is of the nature of an axone reflex. The sensory fibres must be 

 supposed to branch at the periphery, one part supplying receptors in the skin, the 

 other supplying the muscular coat of arterioles and acting there as efferent in- 

 hibitory fibres. When the receptor branch is stimulated, excitation spreads to the 

 main fibre, and back along the vascular branch to the blood vessels. It may be 

 that this latter enters a peripheral network, and it is not impossible that the par- 

 ticular receptors stimulated by the irritant substance may also form some part of 

 this network (see the diagram in Fig. 145). 



THE SYNAPTIC SYSTEM 



The anatomical unit of the higher central nervous systems is, as we have seen, 

 the neurone. Perhaps the clearest proof of the structural discontinuity of the 

 individual neurones is afforded by the fact that the degeneration which takes place 

 in a nerve fibre, when it is cut off from the rest of its neurone, only proceeds as far 

 as its contact (synapse) with the processes (dendrites) of another neurone. 

 Although physiological continuity must exist, there is evidently an absence of 



protoplasmic or nutritive con- 

 tinuity. As Sherrington 

 points out (1906, p. 17), such 

 a contact surface is of great 

 functional importance since 

 " it might restrain diffusion, 

 back up osmotic pressure, 

 restrict the movement of ions, 

 accumulate electric charges, 

 support a double electric layer, 



FIG. 145. DIAGRAM OF REFLEX AKTIDROMIC VASCULAR alter in shape and surface 

 DILATATION AS AXONE REFLEX. tension with changes in differ- 



ence of potential, alter in 



A stimulus at A gives rise to an impulse passing along fibre B to the i-. < . i -.1 



spinal cord. A branch from this fibre A is given off at C, which Qinerence OI potential With 

 ends in the walls of the arteriole D. Stimulation of the sensory /.Vionfroc! in ^m-t'-i,-,. fnneirn rr 



c! in ^m-t'-i,-,. fnneirn 



end organ of the fibre at A gives rise to an impulse which passes 

 to the arteriole along the branch C in addition to reaching the in shape, Or intervene as a 



spinal cord, if the main fibre B is inUct membrane between dilute solu- 



(Bainbridge and Menzies, " Essentials tions of electrolytes of different 



of Physiology," p. 289.) concentration or colloidal 



suspensions with different sign 



of charge " ; in fact, all the numerous phenomena which the earlier chapters of the 

 present book have shown to be of fundamental importance in the mechanism of 

 the cell. We have also manifold possibilities of excitation and inhibition in the 

 use of one and the same neurone in different nervous acts and the consequent 

 advantages to the organism in economy of machinery. 



Some points regarding the properties of the synaptic membrane have been 

 already alluded to, but may be recapitulated here. 



Since there is no structural continuity, the possibility of actual retraction owing 

 to increase of surface tension must be admitted. 



The action of electrolytes (page 218), chloroform, and strychnine (page 427) 

 is, no doubt, exercised on the synaptic membrane, which, like other cell membranes, 

 is, presumably, a colloidal system. 



The summation of a series of ineffective stimuli, so that a reflex is ultimately 

 produced, is a common property of nerve centres. Also "facilitation," as it is 

 called by Sherrington, in which an effective stimulus leaves the mechanism for a 

 time capable of excitation by stimuli which were previously too weak, seems to 

 be a further aspect of the same phenomenon. The work of Adrian and Lucas 

 (1912, p. 121) has been already referred to. 



Adrian's work (1912, p. 411) on the "all or nothing" principle has important 



