THE CIRCULATION 265 



of the vasodilator fibres seems to be increased. Thus, if, 

 when the lirnb is warm, the sciatic nerve is stimulated, 

 dilatation rather than constriction may occur. Again, while 

 rapidly repeated and strong induction shocks are apt to 

 cause constriction, slower and weaker stimuli tend to pro- 

 duce dilatation. 



Course. The vaso-dilator nerves pass out by the anterior 

 roots of the various spinal nerves, and do not pass through 

 the sympathetic ganglia, but run as medullated fibres to 

 their terminal ganglia (Fig. 126). 



3. Portions of Nervous System Presiding over the Yaso- 

 motor Mechanism. Since a set of nerves causing constriction 

 of the arterioles, and another set causing dilatation exist, 

 we must conclude that there are two mechanisms in the 

 central nervous system, one a vaso-constrictor, the other a 

 vaso-dilator. 



A. Yaso- constrict or Centre. (a) Mode of Action. This 

 mechanism is constantly in action, maintaining the tonic 

 contraction of the arterioles. 



If any afferent nerve be stimulated, the effect is to increase 

 the activity of the mechanism, to cause a general constriction 

 of arterioles, and thus to raise the general arterial pressure. 

 It is, therefore, capable of reflex excitation. In ordinary 

 conditions so many afferent nerves are constantly being 

 stimulated, that it is not easy to say how far the tonic action 

 of this centre is reflex and dependent on the stream of 

 afferent impulses. 



But this centre may also be directly acted upon by the 

 condition of the blood and lymph circulating through it. 

 When the blood is not properly oxygenated, as in asphyxia 

 or suffocation, this centre is stimulated and a general con- 

 striction of arterioles with high blood pressure results. 



(6) Position. In investigating the position of the centre 

 we may take advantage of 



1st. Its constant tonic influence. Removal of the centre 

 at once causes dilatation of arterioles. 



2nd. The fact that it may be reflexly stimulated. If the 

 vaso-constrictor centre be removed, stimulation of an afferent 

 nerve no longer causes constriction of the arterioles. 



