210 VASO-MOTOR NERVES. [BOOK i. 



inenter be made to produce either arterial dilation or arterial con- 

 striction. 



In all the above cases section of the nerve produces dilation, 

 whether the subsequent stimulation causes constriction or dilation; 

 the dilation after section may be sometimes not very marked, but 

 is always present to some extent or other. But there are certain 

 nerves, section of which produces no marked changes in the vascular 

 areas to which they are distributed, and yet stimulation of which 

 brings about dilation often of an extreme character. A striking 

 example of this is seen in the so-called nervi erigentes. The 

 erection of the penis is, putting aside the subsidiary action of 

 muscular bands in restraining the outflow through the veins, chiefly 

 due to the dilation of branches of the pudic arteries, whereby a 

 large quantity of blood is discharged into the venous sinuses. 

 Erection may in the dog be artificially produced by stimulating the 

 peripheral ends of the divided nervi erigentes, which are branches 

 from the first and second and sometimes from the third sacral nerve 

 passing across the pelvis. On applying the interrupted current to 

 the peripheral ends of these nerves, the corpora cavemosa at once 

 become turgid. And yet simple section of these nervi erigentes 

 will not in itself give rise to erection. 



A similar case is presented by the submaxillary gland. As will 

 be explained more in detail in treating of secretion, this gland 

 is supplied by two nerves, by branches of the chorda tympani 

 reaching it along its duct, and by branches of the cervical sym- 

 pathetic reaching it along its arteries. Neither section of the 

 chorda tympani nor section of the cervical sympathetic produces 

 any very marked effect in the circulation of the gland. Yet stimu- 

 lation of the former will bring about a most striking dilation, of 

 the latter a no less striking constriction, of the arteries of the gland. 



How can we construct a view of the action of vaso-motor nerves 

 which will be consistent with all these various facts? 



In the first place, we must admit the existence of a local tone 

 in the several vascular areas, independent of the central nervous 

 system. In such cases as the corpora cavernosa of the penis,, and 

 the submaxillary gland, this independence is unmistakeable ; in 

 other regions it is not at first sight so apparent, but, as we have 

 already urged, must be admitted even for these. 



In the second place, as is strikingly shewn by the case of the 

 submaxillary gland, there are nerves which, since stimulation of 

 them always causes dilation, may be called vaso-dilator nerves, and 

 nerves which, since stimulation of them always causes constriction, 

 may be called vaso-constrictor nerves. Examples of the first are 

 seen in the nervi erigentes, the chorda tympani, the nerves of 

 muscles, &c. ; of the second, in the cervical sympathetic, the 

 splanchnic, &c. Or to be more exact, we may say that the vaso- 

 motor fibres of the former are vaso-dilator, of the latter, vaso-con- 

 strictor. 



