THE CIRCULATION OF THE BLOOD AND LYMPH 151 



dilatation of the vessels. This experiment can be very 

 readily performed on the rabbit, and the changes are most 

 easily followed in an albino. The ear on the side of the cut 

 nerve is redder and hotter than the other ; the main arteries 

 and veins are swollen with blood, and many vessels formerly 

 invisible come into view. The slow rhythmical changes of 

 calibre, which in the normal rabbit are very characteristically 

 seen in the middle artery of the ear, disappear for a time after 

 section of the sympathetic, although they ultimately again 

 become visible (Practical Exercises, p. 190). 



Stimulation of the cephalic end of the cut sympathetic 

 causes a marked constriction of the vessels and a fall of 

 temperature on the same side of the head. From these 

 facts we know that the cervical sympathetic in mammals 

 contains vaso-constrictor fibres for the side of the head and 

 ear, and that these fibres are constantly in action. Certain 

 parts of the eye, and the salivary glands, larynx, oesophagus, 

 and thyroid gland, are also supplied with vaso-motor (con- 

 strictor) nerves from the cervical sympathetic. 



It has been asserted that the cervical sympathetic con- 

 tains some of the vaso-constrictor fibres that supply the 

 corresponding half of the brain and its membranes, but this 

 has been disputed, and some observers have even gone so far 

 as to deny that the vessels of the brain have any vaso-motor 

 nerves. Non-medullated nerve-fibres, however, may be seen 

 in and around the walls of the cerebral bloodvessels (Huber), 

 and it is difficult to believe that these have not a vaso-motor 

 function, although this has not as yet been clearly demon- 

 strated by experimental methods. 



That the nerve contains some dilator fibres is proved 

 by the fact that stimulation of the cephalic end in the 

 dog causes flushing of the mucous membrane of the mouth 

 on the same side. The vaso-motor fibres of the head run 

 up in the cervical sympathetic, 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 



