88 VASCULAR NERVOUS SYSTEM. 



opposite effect is produced (see below). It cannot be 

 stated whether in this case the constrictor centres are 

 acted upon directly, or with the intervention of other 

 centres. I. Section of the spinal cord in the neck causes in 

 all animals vascular dilatation and consequent diminution 

 of the arterial pressure, and of the velocity of the circula- 

 tion. ' As the dilatation affects the vessels of the viscera 

 much more than those of the skin and of the muscles, 

 the distribution of the blood is altered. 2. Excitation of 

 any external sensory nerve produces contraction of the 

 blood vessels of the viscera, but dilatation of those dis- 

 tributed to the muscles and skin (Heidenhain, Bernstein). 

 In the normal animal the effect of these vascular changes 

 is to increase the velocity of the circulation and the 

 arterial pressure. In those animals in which the depres- 

 sor forms a separate nerve, excitation of the central end 

 of that nerve produces dilatation of the visceral blood 

 vessels, and consequent diminution of arterial pressure. 

 3. Severance of the constrictor nerves distributed to 

 external parts, or of spinal nerves, produces relaxation 

 of the arteries to which they are distributed, but after a 

 time the arterial tonus (see below) is restored, notwith- 

 standing that the communication between the arteries 

 and the central nervous system continues to be inter- 

 rupted. Excitation of the same nerves (peripheral ends 

 after section) determines, under normal conditions vas- 

 cular constriction, pallor, and diminished temperature, in 

 the parts to which they are distributed, and diminishes 

 the flow of blood in the veins which lead from those parts. 

 If, however, the nerves subjected to excitation are in a 

 state of partial degeneration, consequent on previous 

 severance, it often happens that the opposite effects are 

 produced. Again, if the temperature of the part is 

 already lower than the normal, the vessels dilate in 

 response to the excitation instead of contracting, even 

 though the nerve excited may have been divided imme- 



