1 66 A MANUAL OF PHYSIOLOGY 



affected. The question whether efferent fibres are connected with 

 nerve-cells between a given point and their peripheral distribution 

 can, therefore, be answered by observing whether any effect of stimu- 

 lation is abolished by nicotine. If, for instance, the excitation of a 

 nerve caused constriction of certain bloodvessels before, and has no 

 effect after, the application of nicotine to a ganglion, its vaso-con- 

 strictor fibres, or some of them, must be connected with nerve-cells 

 in that ganglion. Langley has recently brought forward evidence 

 that many of the bodies which are commonly supposed to act upon 

 nerve-endings (as nicotine, curara, atropine, pilocarpine, adrenalin, 

 etc.) really act upon ' receptive ' substances of the cells in connec- 

 tion with which the nerve-fibres end. These receptive substances 

 are conceived to be capable of being specifically affected by chemical 

 bodies and by nervous stimuli, and in their turn to be capable of 

 influencing the metabolism of the main cell substance on which its 

 function depends. The receptive substances thus form beyond the 

 histological link of the nerve-ending a kind of chemical link between 

 the nerve-fibre and the cell which it supplies. 



We have thus traced the vaso-motor nerves from the cerebro- 

 spinal axis to the bloodvessels which they control ; it still remains 

 to define the portion of the central nervous system to which these 

 scattered threads are related, which holds them in its hand and 

 acts upon them as the needs of the organism may require. 



Vaso-motor Centres. Now, experiment has shown that there 

 is one very definite region of the spinal bulb which has a most 

 intimate relation to the vaso-motor nerves. If while the blood- 

 pressure in the carotid is being registered, say, in a curarized 

 rabbit, the central end of a peripheral nerve like the sciatic 

 is stimulated, the pressure rises so long as the bulb is intact, 

 this rise being largely due to the reflex constriction of the vessels 

 in the splanchnic area. If a series of transverse sections be made 

 through the brain, the rise of pressure caused by stimulation of 

 the sciatic is not affected till the upper limit of the bulb is almost 

 reached. If the slicing is still carried downwards, the blood- 

 pressure sinks, and the rise following stimulation of the sciatic 

 becomes less and less. When the medulla has been cut away to 

 a certain level, only an insignificant rise or none at all can be 

 obtained. The portion of the medulla the removal of which 

 exerts an influence on the blood-pressure, and its increase by 

 reflex stimulation, extends from a level 4 to 5 mm. above the 

 point of the calamus scriptorius to within i to 2 mm. of the 

 corpora quadrigemina. Stimulation of the medulla causes a 

 rise, destruction of this portion of it a severe fall, of general 

 blood-pressure. There is evidently in this region a nervous 

 ' centre ' so intimately related, if not to all the vaso-motor 

 nerves, at least to such very important tracts as to deserve the 

 name of a vaso-motor centre. Experiment has shown that this 

 is much the most influential centre, and it is usually called the 

 chief or general vaso-motor centre. Some writers prefer to 



