Bauhaus-Universität Weimar

Handbook for the Physiological Laboratory. Text
Burdon-Sanderson, John Scott E. Klein Michael Foster T. Lauder Brunton
(25°—35° C.) may be used instead of the acid. It has the advantage 
of being less likely than the acid to produce a permanent action on 
the skin. The difficulty, however, of keeping up exactly the same 
temperature renders it unsuitable for comparative experiments. 
In all the above experiments the movements produced bear 
marks of purpose. As the result of stimulation of a particular 
region of the surface of the body, we find a complicated move« 
ment, a movement brought about by the contraction of certain 
muscles and sets of muscles, acting in a definite combination and 
sequence. The movement thus produced is apparently directed 
towards an end. Thus when the foot is pinched or irritated by 
the acid, the resulting movements appear at least directed towards, 
and frequently actually effect, the withdrawal of the foot from the 
offending object ; when the flank is pinched, the movement is such 
as tends to thrust away the points of the forceps ; when the anus 
is pinched to kick away the forceps, and so on. 
This purposeful character of reflex actions may be still more 
conveniently shown by adopting the following method :— 
Obs. VIII. Arrange the frog with the legs alone free accord« 
ing to the second method given above. Cut small pieces of 
blotting-paper about 1 or 2 millimetres square, dip them in strong 
acetic acid, remove from them all superfluous acid, and then place 
them on definite regions of the skin. In this way the stimulus may 
be limited to very small areas chosen at pleasure ; and it will be 
found that very different movements of the hind limbs will be pro¬ 
duced by applying the morsel of paper to different regions of the 
body. Thus, if the morsel be placed on the heel of one foot, both 
feet will be violently rubbed together, while the legs remain forcibly 
extended. If the morsel be placed on one flank, it will be rubbed 
off by the foot of the same side; if it be placed in the mid-line of 
the back, either or both feet will be employed to remove it, and 
so on. 
The student will do well to map out the limbs and body of the 
frog into small areas, and to determine the characters of the 
movements, which result from the stimulation of each area. He 
will in this way find abundant instances of an apparent purpose. 
Obs. IX. It has been seen that where the morsel of acid paper 
is placed, say on the right flank, it is the right leg, and the right 
leg only, which under ordinary circumstances is used to rub off 
the paper. Choosing a strong frog, in which reflex action has been 
found to be highly developed, suspend it according to the second 
method, hold the right leg firmly down, or load it with a greater 
weight than the leg is able to lift, and apply a morsel of acid paper 
to the right flank. Twitchings and convulsive movements of the 
right leg are first witnessed, and then the left leg is brought up 
to rub the right flank.


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