VASOMOTOR ACTIONS. 213 



which case its muscular fibres are relaxed ; or it may be moderately con- 

 stricted, the muscular fibres being contracted to a certain extent, and 

 remaining in such a condition that they may, on the one hand, pass into 

 stronger contraction, leading to marked constriction, or, on the other hand, 

 into distinct relaxation, leading to dilatation. We have reason to think, as 

 we shall see, that many arteries of the body are kept habitually, or at least 

 for long periods together, in this intermediate condition, which is frequently 

 spoken of as tonic contraction, or tonus, or arterial tone. 



151. If, now, in a vigorous rabbit, in which the heart is beating with 

 adequate strength and the whole circulation is in a satisfactory condition, 

 the cervical sympathetic nerve be divided on one side of the neck, remark- 

 able changes may be observed in the bloodvessels of the ear of the same side. 

 The arteries and veins widen, they together with the small veins and the 

 capillaries become full of blood, many vessels previously invisible come into 

 view, the whole ear blushes, and if the rhythmic changes described above 

 were previously going on, these now cease ; and, in consequence of the extra 

 supply of warm blood, the whole ear becomes distinctly warmer. Now these 

 changes take place, or may take place, without any alteration in the heart- 

 beat or in the general circulation. Obviously the arteries of the ear have, 

 in consequence of the section of the nerve, lost the tonic contraction which 

 previously existed ; their muscular coats, previously somewhat contracted, 

 have become quite relaxed, and whatever rhythmic contractions were pre- 

 viously going on have ceased. The more marked the previous tonic 

 contraction, and the more vigorous the heart-beats, so that there is an 

 adequate supply of blood to fill the widened channels, the more striking 

 the results. Sometimes, as when the heart is feeble, or the pre-existing 

 tonic contraction is slight, the section of the nerve produces no very obvious 

 change. 



If, now, the upper segment of the divided cervical sympathetic nerve 

 that is, the portion of the nerve passing upward to the head and ear be 

 laid upon the electrodes of an induction machine and a gentle interrupted 

 current be sent through the nerve, new changes 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 Jhe 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 finding 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 constricted 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 manipu- 

 lation 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 



