216 PRACTICAL PHYSIOLOGY 



experiment is repeated with acid of the strength 1 in 500. In each case 

 the time of response is much longer than the true time of a reflex action. 



The Action of Strychnine and of Chloroform. The cerebrum of a 

 frog is destroyed by means of Spencer Wells' forceps, and then under 

 the skin of the back are injected 10 minims of a saturated solution of 

 strychnine (1 in 6700). In two or three minutes it will be noticed 

 that the frog cannot readily recover its hind legs after a jump, and 

 soon the reflex excitability of the spinal cord is so augmented that a 

 slight touch or puff of wind upon the skin causes a general spasm of the 

 muscles. Convulsions quickly follow, and the rigid body of the frog 

 rests on the mouth and toes, a position known as emprosthotonus. This 

 attitude is due to the different strength of the various muscles ; all are 

 thrown into contraction, but the stronger overcome the weaker. The 

 muscles are somewhat relaxed after the spasms, but are again sent 

 into tetanus by the slightest touch applied to the skin. 



The tonic contractions are followed by prolonged twitches or clonus. 



If during the stage of convulsions a probe be pushed down the 

 vertebral canal, and thus the spinal cord be destroyed, the convulsions 

 cease at once, showing that the strychnine acts upon the ganglion 

 cells and their dendrites in the spinal cord. (See page 220.) 



The action of strychnine should be contrasted with that of chloro- 

 form. Under the skin of the back of a frog, whose cerebrum has 

 been destroyed by Spencer Wells' forceps, are injected 5 minims of 

 chloroform. The first effect is one of stimulation, but this stage of 

 excitement is quickly followed by marked inco-ordination and weak- 

 ness. In about ten minutes there is marked anaesthesia, paralysis, 

 and total absence of reflexes. If the frog be kept moist in a shallow 

 plate full of water, and covered by a bell jar, it may recover from 

 the effects of the chloroform in about eight or nine hours. 



CHAPTER LX. 

 REACTION TIME. 



THE time which elapses between the application of a given stimulus 

 and the prearranged response of the subject to that stimulus is known 

 as the reaction time. It is obviously more complex than a reflex 

 action; this will be readily understood from a consideration of the 

 following determination of the reaction time. 



The diagram 205 shows W. Gr. Smith's reaction time apparatus as 

 modified by Colls. The electro-magnetic tuning fork, T, with 100 



