THE NERVOUS SYSTEM 459 



system, the peristaltic movements of the intestines, the contrac- 

 tion of the bladder and uterus, and the secretions from various 

 glands, are all examples of reflex acts. 



Reversal of Reflex Effect. — The influence of the nervous system in 

 regulating the calibre of the bloodvessels as a reflex effect has been 

 dealt with elsewhere (pp. 57, 58). It is not proposed to refer again to 

 this, excepting to illustrate a phenomenon known as reversal of 

 reflex effect. When an ordinary afferent nerve is stimulated in the 

 dog, the effect produced on the bloodvessels is that of constriction, 

 and the blood-pressure is reflexly raised. Bayliss has shown that a 

 rise in pressure can be converted into a fall by a sufficient dose of 

 chloroform. In observations by Sherrington and Sowton on 

 skeletal muscle, it was found that the excitatory reflexes brought 

 about by strychnine could be undone by chloroform or ether, so that 

 the reflex response could be changed back from contraction to 

 inhibitory relaxation, and on allowing the chloroform narcosis to 

 partially pass off, the strychnine influence reappears. In the de- 

 cerebrate preparation the above observers showed that reflex con- 

 traction of skeletal muscle or reflex inhibition could be produced at 

 will through the same afferent nerve by varying the strength and 

 form of the electrical stimulation, and in other ways. Instances 

 begin to be numerous in which the reflex obtainable from one and 

 the same afferent nerve may be diametrically reversed on alteration 

 of some one definite factor in the conditions of the reaction. 



Peripheral Reflex Centres — viz., centres for reflex action out- 

 side the brain and spinal cord — have long been carefully looked 

 for, especially in connection with the sympathetic system. So 

 far as is known, there are no purely sympathetic reflex acts ; 

 nor are all the necessary physiological structures present in 

 the sympathetic system for the formation of a reflex arc. The 

 ganglia on the dorsal roots of the spinal nerves are similarly 

 structured ; they are not seats of reflex action, and for the same 

 reason. We have seen that a reflex arc requires a sensory nerve, a 

 centre, and a motor nerve. All three of these are not present 

 in connection with a sympathetic or spinal ganglion. 



The Time occupied by a Reflex Act varies dependency upon the 

 strength of the stimulus and the nature of the reflex ; the sharper 

 the stimulus, the more immediate the reflex ; the more active the 

 centre, the more rapid the response. Impulses which have to 

 cross the cord take longer than those which enter and return 

 from the same side. It is mainly during this appreciable delay, 

 or latent period, as measured by delicate apparatus, that the 

 changes are occurring in the grey substance which lead to an 

 efferent response. In the dog the time occupied by a reflexion 

 the same side is estimated at 0*022 up to 2-3 seconds, according 

 to circumstances. 



