160 A MANUAL OF PHYSIOLOGY 



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, dis- 

 appear for a time after section of the sympathetic, although they 

 ultimately again become visible (Practical Exercises, p. 202). 



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 (constrictor) nerves from the cervical sym- 

 pathetic. 



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 

 observers 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 experi- 

 mental 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 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 



