THE VASO MOTOR MECHANISM. I35 



always appears ; if the current be very weak, primary vase-dilatation 

 usually occurs. 



6. On cooling the sciatic nerve to 3 C., the vaso-constrictor fibres are 

 paralysed before the dilator. 1 



For the vaso-dilator nerves, the latent period (between excitation 

 and result) is usually longer than it is for the vaso-constrictor fibres. 

 In the case of the hind-limb, the vaso-dilator latent period is about 

 3*5 seconds, the vaso-constrictor 1-5 seconds. The latent period of the 

 nervi erigentes is recorded as being 34 to 4'0 seconds. 2 Likewise, the 

 maximal dilator effect is reached much more slowly, and dies away 

 much less rapidly than the maximal constrictor effect. 



To sum up, then, the vaso-dilator fibres are more easily excited by 

 an infrequent rate and weak form of stimulus. The vaso-constrictors 



FIG. 83. Plethysmograms of hind-limb. Excitations of sciatic nerve. A, Sixteen 

 per second. B, Sixty-four per second. Bovvditch and Warren. 



degenerate more rapidly after division, and are paralysed more easily 

 by cold. The vaso-dilator effect appears after a longer latent period, 

 while it reaches its maximum and disappears more slowly. 



Hypotheses on the action of vaso-dilator nerves. It is easy to 

 imagine that, excited by the influence of the vaso-constrictor nerves, 

 the muscular coats of the arterioles should constrict the calibre of these 

 vessels. It is a more difficult task to explain by what method the vaso- 

 dilator nerves work their effect. 



The hypothesis that first suggests itself is that the vaso-dilator 

 inhibit the tonic action of the vaso-constrictor fibres. Before accepting 

 this hypothesis, we are confronted with the difficulty that the vessels of 

 any organ are dilated to a greater degree by excitation of the vaso- 

 dilators than they are by paralysis of the vaso-constrictors. In spite of 

 the destruction of the vaso-constrictor nerves, the vessels, after a certain 

 period, recover their tone. That the tone is not even for a time abolished 

 completely, is shown by the fact that a local dilatation can be produced 

 by certain chemical irritants. Thus it is clear that some peripheral 

 mechanism exists, a mechanism which, in its turn, can be paralysed 

 by the local application of such a drug as chloroform. 



1 Howell, Budgett, and Leonard, Journ. PhysioL, Cambridge and London, 1894, vol. vi. 

 p. 306. 



2 Bowditch and Warren, loc. cit., p. 440 ; v. Anrep and Cybulski, Jahresb. ii. d. Fortschr. 

 d. Anat. u. PhysioL, Leipzig, 1884, S. 53. 



