152 A MANUAL OF PHYSIOLOGY 



The lingual branch of the trigeminus supplies vaso-motor 

 fibres to the tongue, and apparently both vaso-constrictor 

 and vaso-dilator. 



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

 part of its vaso-motor supply directly from the cerebro- 

 spinal system, through the great auricular nerve, as well as 

 through the cervical sympathetic. 



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 underfilled, 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. The presence of dilator fibres (for the 

 intestines and the kidney, for example) has also been 

 demonstrated in the splanchnic nerves, although the con- 

 strictors 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 complicated 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- 

 constrictors 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, 

 stimulation by paling and fall of temperature. By taking 

 advantage, however, of the unequal excitability of dilators 



