416 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



extremities, but as this effect is inconstant and very localized, it may 

 have an indirect cause. Moreover, while Henderson^ has found that 

 strips of veins react toward solutions of adrenalin in the same man- 

 ner as segments of arteries, this evidence cannot be considered as a 

 direct proof of the existence of vasomotor elements in the veins. 

 On the whole, therefore, this question seems to have found a negative 

 solution. 



As far as the capillaries are concerned, it has been shown by Strieker 

 and others^ that these tubules possess a certain degree of contractility, 

 but it appears that this reaction cannot acquire a definite dynamical 

 value. All living substance exhibits this property and hence, it can- 

 not be denied to the living cells of the capillaries. Stimuli brought to 

 bear upon them must result in a rearrangement of their contents 

 and a possible constriction of the lumen of the capillary. This re- 

 action, however, does not seem to be of central origin, but appears to 

 be elicited solely by local excitations. In this connection attention 

 should also be called to the fact that the capillary blood-bed may be 

 materially altered by variations in the tension of the surrounding 

 tissues. Thus, the lumen of these tubules may be compressed in 

 consequence of the contraction of the numerous smooth muscle cells 

 which are widely scattered through the skin. The relaxation of these 

 muscular elements, on the other hand, must tend to widen the capillary 

 blood-bed and to grant a more copious blood-supply to the cutaneous 

 parts. Reactions of this kind result in consequence of variations in 

 the temperature of the surrounding air as well as in consequence of 

 the immersion of the body in cold or warm water. The influence of 

 these muscular elements upon the injection of the cutaneous capil- 

 laries can scarcely be overestimated. It should be emphasized, how- 

 ever, that we are not dealing in this case with a true vasomotor 

 phenomenon, but solely with a direct mechanical action. At the 

 same time it must be granted that any influence causing a contrac- 

 tion of the cutaneous smooth muscle tissue, would be prone to 

 produce a vasoconstriction in addition. A reverse relationship, how- 

 ever, need not exist. 



In view of the evidence here presented, it seems permissible to 

 conclude that true vasomotor actions are possible only in the arterial 

 system. Since the smooth muscle tissue is most massive in the arteri- 

 oles, it may be surmised that the most powerful effects of this kind are 

 obtained at the arteriocapillary junction. This segment of the arterial 

 system, therefore, gives lodgment to the gate or sluice through which 

 the blood must pass in order to reach the capillaries. Consequently, 

 the size of this orifice must determine the volume of the arterial escape 

 as well as the vascularity of the more distant capillary networks. 

 Excepting, therefore, certain local influences in the shape of the cuta- 

 neous smooth muscle cells, the caliber of the latter is determined 



1 Am. Jour, of Physiol., xxiii, 1909, 345. 



^ Steinach and Kahn, Pfliiger's Archiv, xlvii, 1903. 



