THE CIRCULATION OF THE BLOOD AND LYMPH 161 



the vessels of the ear whose nerve is intact may become still more 

 dilated than those whose constrictor fibres have been paralyzed. 

 The only explanation is that vaso-dilators are being excited from 

 the central nervous system. In the cat the cervical sympathetic 

 contains vaso-dilators for the submaxillary gland (p. 367). 



The vaso-motor fibres of the head run up in the cervical sym- 

 pathetic, and then pass into various cerebral nerves, of which the 

 fifth or trigeminus is the most important. 



The trigeminus nerve contains vaso-constrictor nerves for 

 various parts of the eye (conjunctiva, sclerotic, iris), and for 

 the mucous 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 apparently 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 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 con- 

 striction of vessels in the intestinal area. We therefore conclude 

 that in the splanchnics there are vaso-motor fibres of the con- 

 strictor 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 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 cer- 

 tainly 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 con- 

 ditions preponderate, so that section of the sciatic or the brachial 

 is generally followed by flushing of the balls of the toes and rise 



ii 



