CHAP, iv.] THE VASCULAR MECHANISM. 309 



take place in the blood vessels of the ear. A short time after the 

 application of the current, for in this effect there is a latent period 

 of very appreciable duration, the ear grows paler and cooler, many 

 small vessels previously conspicuous become again invisible, the 

 main artery shrinks to the thinnest thread, and the main veins 

 become correspondingly small. When the current is shut off from 

 the nerve, these effects still last some time but eventually pass 

 off ; the ear reddens, blushes once more, and indeed may become 

 even redder and hotter, with the vessels more filled with blood 

 than before. Obviously the current has generated in the cervical 

 sympathetic nerve impulses which, passing upward to the ear and 

 rinding their way to the muscular coats of the arteries of the ear, 

 have thrown the muscles of those coats into forcible contractions, 

 and have thus brought about a forcible narrowing of the calibre of 

 the arteries, a forcible constriction. Through the narrowed con- 

 stricted arteries less blood finds its way, and hence the paleness 

 and coldness of the ear. If the impulses thus generated be very 

 strong the constriction of the arteries may be so great that the 

 Smallest quantity only of blood can make its way through them 

 and the ear may become almost bloodless. If the impulses be 

 weak the constriction induced may be slight only ; and indeed by 

 careful manipulation the nerve may be induced to send up to the 

 ear impulses only just sufficiently strong to restore the moderate 

 tonic constriction which existed before the nerve was divided. 



We infer from these experiments that among the various nerve 

 fibres making up the cervical sympathetic, there are certain fibres 

 which passing upwards to the head become connected with the 

 arteries of the ear, and that these fibres are of such a kind that, 

 impulses generated in them and passing upw r ards to the ear, lead 

 to marked contraction of the muscular fibres of the arteries, and 

 thus produce constriction. These fibres are vaso-motor fibres for 

 the blood vessels of the ear. From the loss of tone, so frequently 

 following section of the cervical sympathetic, we may further infer, 

 that, normally during life, impulses of a gentle kind are continually 

 passing along these fibres, upwards through the cervical sympathetic, 

 which impulses, reaching the arteries of the ear, maintain the 

 normal tone of those arteries. But, as we said, the existence of this 

 tone is not so constant, and these tonic impulses are not so 

 conspicuous as the artificial constrictor impulses generated by 

 stimulation of the nerve. 



166. The above results are obtained whatever be the region 

 of the cervical sympathetic which we divide or stimulate from the 

 upper cervical ganglion to the lower. We may therefore describe 

 these vaso-motor impulses as passing upwards from the lower cer- 

 vical ganglion along the cervical sympathetic, to the upper cervical 

 ganglion, from which they issue by branches which ultimately find 

 their way to the ear. But these impulses do not start from the 

 lower cervical ganglion ; on the contrary, by repeating the experi- 



