1'Jl THE XERVOCS SYSTEM AM) ITS ( '< >\SEK Y.\TI<i\ 



heart and to subtract from the pressure in those above the 

 level of that origan. The actual pressure of the blood in 

 any vessel is the resultant of a certain quantity referable 

 to the energy of the heart-beat plus or minus a quantity 

 due to the elevation of the locality as compared \\'ith that 

 of the heart. If the body were always erect, the situation 

 might be met, one may suppose, by the development of 

 thicker and less yielding walls in the ve-sels of the lower 

 extremities, but, in reality, there has to be provision for 

 changes from the hori/ontal to the upright position, and 

 even for bringing the head to occupy the lowest place. 



When a man rises from his bed he does not normally 

 feel any distinct faint ness or dizziness as he would if there 

 were a serious impairment of blood-supply to his brain. 

 Yet the tendency for the blood to forsake the higher arcs 

 of the circulation and to distend the lower ones must be a 

 very positive force. It is manifest when there is actual 

 faint ness, the individual being comparatively comfortable 

 while lying down, though he may be utterly unable to sit 

 up. We have to conclude that in health the shifting of 

 the blood is prevented with singular efficiency by the 

 establishment of a higher vasomotor tone in the vessels on 

 which the .-idded pros u re falls. This is one of the fir>t 

 functions to fail when disturbing conditions arise; it is 

 the lack of this reaction which "confines one to his bed" 

 when sick. Compensation for the head-down position 

 probably never comes to be perfect. We see even in 

 acrobats of long experience the intense flushing of the 

 face when the head is hanging down, as in swinging by 

 the feet from a trape/e. 



We have seen that the vasomotor system makes im- 

 portant and widespread changes in the distribution of the 

 blood under the influence of changes of the external tem- 

 perature, and that it exercises its powers to guard against 

 injurioii- modifications in distribution when the posture is 

 shifted. We have al>o stated the general principle that 

 this sy-tem dilates (or permits the relaxation of) vessels 

 at any center of unusual activity. It would be easy to 



