1 82 THE CIRCULATION OF THE BLOOD AND LYMPH 



Effect of Nicotine on Nerve-Cells. A method which has been found 

 most fruitful in studying the relations of sympathetic ganglion-cells to 

 the vaso-motor fibres, as well as to the pilo-motor* and secretory fibres 

 which in certain situations are so intricately mingled with them, must 

 here be mentioned. It depends upon the fact that when a suitable 

 dose of nicotine (10 milligrammes in a cat) is injected into a vein, or a 

 solution is painted on a ganglion with a brush, the passage of nerve- 

 impulses through the ganglion is blocked for a time (Langley). The 

 nerve-fibres peripheral to the ganglion are not 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 stimulation 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-constrictor fibres, or some of them, must be con- 

 nected with nerve-cells in that ganglion. Langley has 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 connection 

 with which the nerve-fibres end. These receptive substances are con- 

 ceived 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-Motur 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 stimu- 

 lation 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, destruc- 



* Pilo-motor nerves supply the smooth arrector pili muscles, whose contrac- 

 tion causes the hair to ' stand on end.' 



