By graduating the strength of the acid used to moisten a square millimetre of blotting paper, the following results are obtained: When very weak acid is employed only slight local and unilateral movement is caused. Stronger acid produces a series of reflex movements, spreading to several muscles on both sides of the body. If further strengthened, the movements become violent and more extended until the whole body is thrown into convulsion. The movements spread from the nerve cells to their neighbors, and then to those governing the corresponding muscles of the other side, in which, however, they are less marked than in those of the side stimulated.
Slight stimulation, when of short duration, not sufficient to produce immediate response, may, after a time, give rise to definite reflex action, as if the weak impulses arriving at the nerve cells in the cord were stored up until their sum sufficed to produce a definite reflex movement. This may be seen in animals whose nerve centres are intact, for the cells of more remote parts exercise a kind of checking influence on those in the region receiving the stimulus, and thus the accumulative action (summation) comes more effectively into play. In the human subject, where slight visceral stimulations exist for a long time, this summation may be observed. In some of these cases, even without sensory appreciation of the local excitation, an amount of energy may be accumulated in the gray tracts of the cord, that will bring on the most extensive forms of reflex muscular movement. These movements differ often from the regular coordinated motion resulting at once from skin stimulation. As an example of this may be named the convulsions that occur in young children from the prolonged irritation of intestinal worms, or during the painful period of dentition.