202 VASO-MOTOR NERVES. [BOOK i. 



instrument for determining the flow of blood to the various organs 

 and tissues of the body, and thus becomes a means of indirectly 

 influencing their functional activity. We should accordingly ex- 

 pect to find that the vaso-motor nerves were connected with, and 

 arterial tone regulated by, the central nervous system, in order 

 that the calibre of the arteries of, and the supply of blood sent to, 

 this or that vascular area might be varied according to the varying 

 needs of the economy. And experiment proves this to be the 

 case. 



We stated that section of the cervical sympathetic in the neck 

 causes dilation or loss of tone in the blood-vessels of the head and 

 face. This is true at whatever point of the course of the nerve 

 from the upper to the lower cervical ganglion, both included, the 

 section be made. No such dilation of the vessels of the head and 

 face takes place when the thoracic sympathetic chain is divided 

 anywhere below the upper thoracic ganglion; but dilation does 

 occur after division of certain of the rami communicantes connect- 

 ing the spinal cord with the cervical sympathetic through the 

 lower cervical or upper thoracic ganglion. Hence it is clear that 

 the normal tone of the arteries of the head and face is maintained 

 by influences (whose exact nature we shall study presently) pro- 

 ceeding from the central nervous system, passing through certain 

 rami communicantes (the exact path being somewhat uncertain or 

 possibly not constant) into the cervical sympathetic, and ascending 

 to the head and face by that nerve. In other words, the vaso- 

 motor fibres of the vessels of the head and face may be traced 

 down the sympathetic to the lower cervical ganglion, and thence 

 by rami communicantes into the spinal cord. 



In a similar manner the vaso-motor fibres of the splanchnic 

 nerves governing the mesenteric and other abdominal arteries can 

 also be traced into the spinal cord, as may also those of the sciatic 

 governing the blood-vessels of the hind limb and of the brachial 

 nerves governing those of the fore limb. In fact all the vaso- 

 motor fibres (with certain special exceptions which will be discussed 

 presently) may thus be traced into the spinal cord ; they are all 

 connected with the central nervous system. There is at present 

 some uncertainty in certain cases a.s to the exact manner in which 

 the fibres pass from the spinal cord to this or that nerve, as, for 

 instance, along which nerve-roots the vaso-motor fibres eventually 

 joining the sciatic trunk run, whether they all pass on their way 

 into the abdominal sympathetic or no, and the like ; but these are 

 questions which need not delay us now; in whichever way they 

 may be settled, they do not affect the important fact that in some 

 way or other all vaso-motor fibres spring from the central nervous 

 system, and that (with certain special exceptions) what we have 

 called the normal tone of the various vascular areas is maintained 

 by influences proceeding from the central nervous system. 



Far more important however than the maintenance of a normal 



