South African Claived Frog. 



33 



against any stimulation of the acoustic nerves when the mechanism for giving 

 the stimulus is set in action. 



Two plates of vulcanite, hinged below with lead to form a V, and held on 

 an adjustable stand, are brouglit vip to the foot to be stimulated so that the 

 toes lie in the opening of the V, and the vulcanite plates are closed on them 

 without pressure. Two forms of stimulation are employed. One is innocuous 

 and consists of a tap from a liglit hammer, held on a separate stand and 

 attached to a spring. The head of the hammer, which bears a piece of 

 platinum foil, is arranged to strike the vulcanite plate lying against the frog's 

 toes on the release of an electromagnet by the plate-holder as it travels. On 

 the outer surface of the vulcanite is a piece of platinum foil, and the circuit 

 through the signal galvanometer is closed when the hammer-head comes in 

 contact with this. 



The other form of stimuUis may be termed nocuous, since after several 

 applications it will damage the foot. In t|iis form the hammer is replaced by 

 a spring-vice held on a separate stand, which, on the release of the electro- 

 magnet, forcibly compresses the vulcanite arrangement in which the toes lie. 

 This stimulation has been chiefly used in order to obtain the heteronymous 

 reflex in the decerebrate frog. 



By the use of these arrangements a mechanical stimulus, weak or strong, 

 can be given without moving the foot perceptibly. The hammer-tap is itself 

 of the nature of a slight sudden squeeze, since the foot is not displaced to one 

 side by the blow, but held against the vulcanite plate. The strong squeeze 

 is given in such a way that the foot remains stationary. Thus, there is no 

 change in the position of the wick on stimulation, and consequently no 

 alterations of resistance from this cause in the galvanometer circuit, nor is 

 there any stretching of the muscles. . 



The method employed in the experiments is to attach the wicks of the 

 electrodes to one leg and to lead off from it during the course of the experi- 

 ments. The toes on the same side as the electrodes are first stimulated, then 

 the toes on the opposite side, and so the homonymous and heteronymous 

 reflexes are recorded alternately. Dummy electrode wicks are attached to 

 the opposite leg, to keep the conditions uniform on the two sides. 



It is not proposed in this communication to compare in any detailed way 

 the results obtained in the clawed frog with the findings in Kana or in the 

 mammal. I have not at present access to all the literature on the subject. 



It is not easy to compare the reflexes obtained from a preparation studied 

 when it is thoroughly " normal " with those observed after the administration 

 of drugs or previous cooling to a low temperature, or which have been 

 subjected to dissection immediately before experiment ; nor is it easy to 



VOL. xcii. — B. D 



