308 CHANGES IN CALIBRE OF ARTERIES. [BOOK i. 



repeated at somewhat irregular intervals of a minute more or less. 

 The extent and regularity of the rhythm are usually markedly 

 increased if the rabbit be held up by the ears for a short time 

 previous to the observation. Similar rhythmic variations in the 

 calibre of the arteries have been observed in several places, ex. gr. 

 in the vessels of the mesentery and elsewhere ; probably they are 

 widely spread. 



Sometimes no such variations are seen, the artery remains 

 constant in a condition intermediate between the more extreme 

 widening and extreme narrowing just described. In fact we may 

 speak of an artery as being at any given time in one of three 

 phases. It may be very constricted, in which case its muscular 

 fibres are very much contracted ; or it may be very dilated, in 

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

 rately constricted, 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 con- 

 striction, or on the other hand into distinct relaxation, leading 

 to dilation. 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 fre- 

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



165. 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, remarkable changes may be observed in the 

 blood vessels of the ear of the same side. The arteries and veins 

 widen, they together with the small veins and the capillaries be- 

 come 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 conse- 

 quence 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 conse- 

 quence of the section of the nerve, lost the tonic contraction which 

 previously existed ; their muscular coats previously somewhat con- 

 tracted have become quite relaxed, and whatever rhythmic con- 

 tractions were previously 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 upwards 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- 



