L'eS 



EXPERIMENTAL PHYSIOLOGY 



The tracing also shows one other point of some importance. It is 

 to be noticed that the heights of the break contractions do not show 

 a perfectly uniform gradation, but offer some irregularities. This is 

 due mainly to irregularities in the strength of the stimulus, for the 

 induced shock at break is in reality compound, and caused mainly by 

 the break of the current, and also by the break extra-current which 

 sparks across in quite an irregular manner at the instant the break is 

 effected. 



UNIPOLAR EXCITATION 



Experiment 6. — Set up the coil to give single shocks, and at first only 

 attach one wire to the secondary coil. Excise a nerve muscle preparation, 

 and placing it upon a dry glass plate put the single wire from the secondary 

 coil under the nerve. On opening or closing no contraction occurs. Next 

 insert a second wire in the remaining terminal of the secondary coil and 

 attach its other end to a gas pipe and so to the earth. A contraction will now 

 occur both on opening or closing the primary circuit. 



Thus it is seen that, in the latter case, the amount of current 

 which passes through the earth and the glass plate is sufficient to 

 stimulate the nerve. It is in order to avoid excitation in this way 

 that the Du Bois key is used as a short-circuiting key in the secondary 

 circuit. 



RECORDING MOVEMENTS BY MEANS OF TAMBOURS 



In recording movements of different parts of the body, it is often 

 necessary to be able to transmit that movement to some little distance 

 because the part cannot be conveniently brought sufficiently near to 



Fig. 34 Makey's Form of Recording Tambour. 



the recording surface to be able to write its movements directly upon 

 the surface. When this is the case, one of the most convenient 

 methods is to employ a pair of tambours, one of which is termed the 

 receiving tambour and the other the recording tambour. 



Each tambour consists of a shallow circular metal box whose upper 

 surface consists of a rubber membrane so that it is air-tisrht. A tube leads 



