ADVANCED EXPERIMENTAL PHYSIOLOGY 171 



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 



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 den- 

 drites in the spinal cord. (See Fig. 

 162.) 



The action of strychnine should be con- 

 trasted with that of chloroform. 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 FIG. 1 61. Diagram of the 



stimulation, but this stage of excitement frog's brain. 



is quickly followed by marked inco- i> Olfactory lobe ; 2, cere- 



* ,. J , , J T i brum; 3, pineal gland; 4, thalam- 



ordmation and weakness. In about ten encephaion ; 5, optic lobe ; e, 

 minutes there is marked anaesthesia, SJ'SSffii, 1 j3K mM * > 

 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 chloro- 

 form in about eight or nine hours. 



THE DISCHARGE OF NERVOUS IMPULSES FROM THE CENTRAL 

 NERVOUS SYSTEM 



The discharge of nervous impulses by the central nervous system 

 can be investigated in the frog by exciting the nerve cells by means 

 of a drug such as strychnine and recording the resulting incom- 

 plete tetanus ; or in man by the record of the contraction of a 

 muscle thrown into contraction voluntarily, or involuntarily as in 

 shivering. 



(a) The Incomplete Tetanus produced by Strychnine. The 

 cerebral hemispheres of a frog are destroyed by compression with a 

 pair of small pliers or Spencer Wells' forceps, and then the gastro- 

 cnemius muscle is prepared with the circulation intact. A piece of 

 string is placed under the gastrocnemius muscle and is then tightly 

 tied round the upper portion of the tibio-fibula and the remaining 

 muscles ; the leg is now cut away below the ligature. In this 

 manner haemorrhage is prevented, the circulation in the muscle is 

 intact, and the muscle is free to move with each contraction. A 

 strong pin is placed through the lower extremity of the femur and is 

 pushed firmly into the cork of the myograph ; a piece of moist flannel 

 is pinned down over the body of the frog in order to prevent the 



