Reflex times in the South African clawed frog

This investigation was undertaken with the object of determining the times of certain reflexes in a species of frog which has not, so far as I know, already been used for the purpose, viz., the South African clawed frog or toad (Xenopus lævis , or an allied species), and by analysing these times to obtain a measure of the delay in transmission of the reflex impulses in the spinal cord, or "synapse time." Spinal and decerebrate animals have been used as well as the intact frog. Einthoven's string galvanometer was employed to indicate the beginning of activity in the limb muscles. It seemed desirable to eliminate, as far as possible, everything that would tend to introduce doubt into the interpretation of the records, even at the cost of making the experiments more troublesome to carry out. For this reason no strychnine or other drug was used. The spinal and decerebrate preparations were kept alive before experiment until the wounds were entirely healed and the frogs appeared to be in thoroughly good condition. Records were then taken from day to day, and the influence of temperature and of the period of survival after operation was studied. Any operative procedure at the time of experiment was rendered unnecessary by employing the mechanical stimulus of a tap by a slight hammer to the toes, or a squeeze of the toes, and the electrical changes in the limb muscles were recorded by means of the wicks of non-polarisable electrodes, moistened with salt solution and impregnated with kaolin, bound round the limb over the unbroken skin. The wicks were applied at knee and ankle, or thigh and ankle, the skin area covered by each wick being about 1·5 cm. broad. The advantage of this method of leading off is that there is no injury done to the preparation, which can be experimented with for months. A disadvantage is that one cannot record in this way the pure response of a single muscle nor control exactly the points led off from, and the form of the electrical response is found accordingly to be rather variable. The application of a mechanical stimulus to the toes instead of electrical stimulation of a nerve, besides being more "natural," is advantageous from its avoiding injury to the preparation, and also by avoiding the difficulties which arise in recording and interpreting the electrical response when the stimulus is also electrical. Precautions will prevent an induction shock given to a dissected-out nerve from affecting the galvanometer, but it is often difficult to get rid of some slight action upon the string, and the absence of this complication makes me more confident in interpreting the curves. There is also no danger of the stimulus spreading further than is desired. The disadvantage of the use of a tap or squeeze is that a direct stimulation of muscle or nerve might perhaps be caused, whose electrical expression would be visible on the record and be mistaken for a reflex effect. This is a real difficulty in the case of homonymous stimulation, and the attempt has been made to avoid it by stimulating the toes alone and not the rest of the foot. The difficulty does not arise in the case of the heteronymous reflex where the electrodes are applied to the limb opposite to the one stimulated. Here there is no appearance on the records of anything which could be the expression of a direct excitation of the foot.