THE SYMPATHETIC SYSTEM. 507 



of the blood-vessels and general reversal of the foregoing results. But 

 if either the lingual nerve, or the chorda tympani above its junction, 

 be divided, the effect is a diminution of the circulatory current both in 

 the tongue and submaxillary gland. On the other hand, galvanization 

 of the peripheral extremities of these nerves causes dilatation of the 

 blood-vessels, and all the phenomena of increased circulation. 



It must be admitted, accordingly, that the dilator nerves exert a 

 direct local influence which causes relaxation of the blood-vessels. 

 The mechanism of this influence is not easily understood ; since the 

 only muscular fibres connected with the arteries surround them in a 

 circular direction, and could produce by their contraction no other 

 effect than a narrowing of the arterial tube. The action of the dilator 

 nerves can only be explained as an "action of arrest/' They convey 

 from the nervous centres outward an influence which for the moment 

 interrupts the tonic contraction of the blood-vessels. Owing to this 

 suspension of tonicity, the vessels dilate under the pressure of the 

 blood, and allow it to circulate in larger quantity. When the suspen- 

 sive action is terminated, the normal stimulus of the sympathetic fibres 

 restores the tonicity of the blood-vessels, and the circulation returns to 

 its ordinary condition. 



The action of arrest, as a nervous phenomenon, is not confined to the 

 vascular system. All the sphincters exhibit it in a marked degree. 

 These muscles are habitually in a state of tonic contraction, by which 

 they keep the outlets of the body closed without voluntary effort. 

 But when evacuation of the rectum or bladder is to take place, the 

 first step in the process is an influence proceeding from the spinal cord, 

 which suspends the contraction of the sphincters ; and after their relaxa- 

 tion, the expulsion of the urine or feces is effected by other muscles. 

 Wherever antagonistic muscles exist, it is evident that the contraction 

 of one, to be effective, must be accompanied by the relaxation of its 

 opposite ; and in all voluntary movements, the relaxation of one set 

 of muscles is as prompt and as accurately adjusted as the contraction 

 of the other. It is probable that the action of arrest plays an impor- 

 tant part in the nervous operations generally ; but it is most distinctly 

 manifest in the dilator nerves of the vascular system. 



Reflex Contraction and Dilatation of the Blood-vessels. Thus far 

 the variations in calibre of the blood-vessels have been shown, in exper- 

 imental observations, to depend on the immediate action of the vasomotor 

 and dilator nerves. But in the living body these variations are habit- 

 ually reflex in their mode of production. The vascular contraction or 

 dilatation which shows itself in a particular area, is due to the impres- 

 sion received by a sensitive surface, conveyed inward to some nervous 

 centre of the vasomotor system, and thence reflected in a centrifugal 

 direction to the blood-vessels. The most frequent instance of reflex 

 vascular constriction is that which follows irritation of the central 

 extremity of a sensitive nerve. This effect has been observed by 

 many experimenters, and is regarded as nearly invariable. Galvaniza- 

 tion of the central extremity of the sciatic nerve causes general con- 



