THE NERVOUS REGULATION OF THE BLOODVESSELS 175 



membrane of the nose and gums, and section of it is followed by 

 dilatation of the vessels of these regions. The lingual branch of 

 the trigeminus supplies vaso-motor fibres to the tongue, and ap- 

 parently both vaso-constrictor and vaso-dilator. 



In some animals the rabbit, for instance the ear derives part 

 of its vaso-motor supply through the great auricular nerve, a branch 

 of the third cervical nerve, which they reach as grey rami from the, 

 stellate ganglion. 



Another great vaso-motor tract, the most influential in the body, 

 is contained in the splanchnic nerves, which govern the vessels of 

 many of the abdominal organs. Section of these nerves causes an 

 immediate and sharp fall of arterial pressure. The intestinal vessels 

 are dilated and overfilled with blood. As a necessary consequence 

 of their immense capacity, the rest of the vascular system is under- 

 filled, and the blood- pressure falls accordingly. Stimulation of the 

 peripheral end of the splanchnic nerves causes a great rise of blood- 

 pressure, owing to the constriction of vessels in the intestinal area. 

 We therefore conclude that in the splanchnics there are vaso-motor 

 fibres of the constrictor type, and that impulses are constantly 

 passing down them to maintain the normal tone of the vascular 

 tract which they command. When the splanchnic nerves are 

 stimulated, the adrenal glands are so affected that adrenalin passes 

 out by the veins into the blood-stream. It is clear that if the quan- 

 tity thus liberated were sufficiently large and its liberation suffi- 

 ciently prompt it might play a part in the rise of pressure (p. 640) 

 which follows stimulation of the nerves, whether they are excited 

 directly or in the normal course of events reflexly. But it has not 

 been demonstrated that this is an effective factor. Dilator fibres 

 (for the intestines and the kidney, for example) have also been 

 discovered in the splanchnic nerves, although the constrictors 

 predominate, and special methods have to be employed for the 

 detection of the dilators. 



The same is true of the nerves of the extremities, which certainly 

 contain vaso-dilator fibres in addition to vaso-constrictors, although 

 the difficulty of demonstrating the presence of the former is fully 

 as great as it is in the splanchnics. For the investigation is com- 

 plicated by the fact that such nerves as the sciatic supply with 

 vaso-motor fibres two leading tissues skin and muscle; and these 

 are not necessarily affected in the same direction or to the same 

 extent by stimulation of their vaso-motor fibres. The vaso-con- 

 strictors under ordinary conditions preponderate, so that section of 

 the sciatic or the brachial is generally followed by flushing of the 

 balls of the toes and rise of temperature of the feet, stimulation by 

 paling and fall of temperature. By taking advantage, however, of 

 the unequal excitability of dilators and constrictors in a degenerating 

 nerve, and of the differences between the two kinds of fibres in their 



