CHAP, iv.] THE VASCULAR MECHANISM. 313 



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 

 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, and that there is no need to pre-suppose the 

 existence of any special terminal inhibitory or dilating nervous 

 mechanism. 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 contrac- 

 tion itself; and there is a priori no reason why a nervous impulse 

 should not govern the former as it does the latter. We may 

 perhaps go further and say that relaxation need not be considered 

 as the mere undoing of a contraction, that the action of dilator 

 fibres is not necessarily limited to the removal of a previously 

 existing constriction. We may imagine a muscular fibre as 

 subject to the action of two opposing forces, the one elongating, 

 relaxing, or dilating, the other shortening, contracting, or con- 

 stricting; when neither are in action, or when the two are 

 equipollent, the fibre is at rest, neither relaxing nor contracting ; 

 when one acts alone, or when one acts more powerfully than the 

 other, then either relaxation, elongation, dilation, or contraction, 

 shortening, constriction is the result ; we have probably as much 

 right to suppose relaxation to be a necessary antecedent of con- 

 traction as to suppose contraction to be a necessary antecedent of 

 relaxation. 



168. But we must return to the vaso-motor nerves. The 

 cervical sympathetic contains vaso-constrictor fibres for the ear, 

 and we may now add for other regions also of the head and face. 

 Thus the branches of the cervical sympathetic going to the sub- 

 maxillary gland of which we just spoke, (Fig. 59 n. sym. sm.), 

 contain vaso-constrictor fibres for the vessels of the gland ; stimu- 

 lation of these fibres produces, on the vessels of the gland, an 

 effect exactly the opposite of that produced by stimulation of the 

 chorda tympani. But to this particular point we shall have to 

 return when we deal with the gland in connection with digestion. 

 A more important fact for our present purpose is that the cervical 

 sympathetic appears to contain only vaso-constrictor fibres ; if we 

 put aside as exceptional and doubtful the result of certain ob- 

 servers who obtained vaso- dilator effects in the mouth and face, 



