174 THE CIRCULATION OF THE BLOOD AND LYMPH 



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 

 (constrictor) nerves from the cervical sympathetic. 



It has been asserted that the cervical sympathetic contains some of 

 the vaso-constrictor fibres that supply the corresponding half of the 

 brain and its membranes, but this has been disputed, and some ob- 

 servers 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 and spinal bloodvessels, and it is difficult to believe 

 that these have not a vaso-motor function, although this has not as 

 yet been clearly demonstrated by experimental methods. 



It has sometimes been argued that we ought not to expect the brain 

 to be supplied with vaso-motors. For it is enclosed in a rigid box, and 

 the quantity of blood in it can be increased or diminished only to the 

 slight extent to which the cerebro-spinal liquid can be displaced into 

 the vertebral canal. Important changes in the cerebral blood-supply 

 are therefore brought about, it is said, not by a widening or narrowing 

 of the cerebral vessels, but by an alteration in the velocity of the blood 

 in them as a result of a rise or fall of the systemic arterial pressure. 

 This argument, however, leaves out of account the consideration that 

 in general the brain does not function as a whole, but that certain 

 parts of it may often become active and relatively hyperaemic, while 

 other parts are inactive and relatively anaemic, and that important 

 changes in the distribution of the blood in the encephalon may be 

 effected, although the total mass of blood in the organ undergoes little 

 or no alteration. It is, of course, true that it is not the absolute quantity 

 of blood in an organ which is a function of its activity, but the rate at 

 which it is renewed. And it is theoretically possible that an organ at 

 rest should contain as much blood as when it is active, or even more. 

 But such cases, if they exist, are certainly rare. The fact that adrenalin 

 generally constricts the vessels of a perfused brain (Wiggers) is in favour 

 of the existence of vaso-motors. The retina, which from the stand- 

 point of development is a portion of the brain, is undoubtedly supplied 

 with vaso-constrictor fibres which run in the cervical sympathetic. 



That the cervical sympathetic 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. Further, after division of the nerve on one side in the rabbit 

 it may be observed that when the animal is excited the vessels of the 

 ear whose nerve is intact may become still more dilated than those 

 whose constrictor fibres have been paralyzed. The only explana- 

 tion 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. 386). 



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

 thetic, 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 



