CHAP, iv.] THE VASCULAR MECHANISM. 331 



nerve caused contraction of the muscular fibres, constriction of the 

 small arteries ; here stimulation of the nerve causes a widening of 

 the arteries, which widening is undoubtedly due to relaxation of 

 the muscular fibres. Hence we must distinguish between two 

 kinds of vaso-motor fibres, fibres the stimulation of which produces 

 constriction, vaso-constrictor fibres, and fibres the stimulation of 

 which causes the arteries to dilate, vaso-dilator fibres, the one kind 

 being the antagonist of the other. 



The reader can hardly fail to be struck with the analogy 

 between these two kinds of vaso-motor fibres on the one hand, and 

 the inhibitory and augmentor fibres of the heart on the other 

 hand. The augmentor cardiac fibres increase the rhythm and 

 the force of the heart beats ; the vaso-constrictor fibres increase 

 the contractions of the muscular fibres of the arteries; the one 

 works upon a rhythmically active tissue, the other upon a tissue 

 whose work is more or less continuous, but the effect is in each 

 case similar, an increase of the work. The inhibitory cardiac 

 fibres slacken or stop the rhythm of the heart and diminish the 

 beats; the vaso-dilator fibres diminish the previously existing 

 contraction of the muscular fibres of the arteries so that these 

 expand under the pressure of the blood. 



We must not attempt here to discuss what is the exact nature 

 of the process by which the nervous impulses passing down the 

 vaso-dilator fibres thus stop contraction and induce relaxation ; 

 but we may say that in all probability the process, whatever be 

 its nature, is one which takes place in the muscular fibre itself on 

 the arrival of the nervous impulse. In the case of the vaso- 

 constrictor fibres there is no need to presuppose the existence of 

 any special terminal nervous mechanism to carry out the con- 

 striction of the vessel, that is to say the contraction of the muscular 

 fibres of its coats, over and above that which exists in the case of 

 all motor nerves and the muscular fibres which they govern. And 

 by analogy we have no valid reason to presuppose the existence of 

 any special terminal mechanism for the vaso-dilator fibres. We 

 have repeatedly insisted that the relaxation of a muscular fibre is 

 as much a complex vital process, is as truly the result of the 

 metabolism of the muscular substance, as the contraction itself; 

 and there is d, priori no reason why a nervous impulse should not 

 govern the former much in the same way as it does the latter. 



168. We must return to the vaso-motor nerves. In the 

 chorda tympani, the vaso-motor fibres are exclusively vaso-dilator 

 fibres, and this is true both of the part of the nerve ending in the 

 submaxillary and sublingual glands, and the rest of the ending of 

 the nerve in the tongue. Stimulation of the chorda tympani (so 

 far as the vaso-motor functions of the nerve are concerned, for it 

 has, as we shall see, other functions), at any part of its course from 

 its leaving the facial nerve to its endings in the gland or tongue, 

 produces only vaso-dilator effects, never vaso-constrictor effects. 



