3. Temperature.
Many difficulties surround the testing of sensibility to heat and cold, particularly as minor degrees of temperature play so considerable a part in our investigations. The use of ordinary glass test-tubes is open to serious objection except for the coarsest observations, for the wall of the tube is never at the same temperature as the fluid it contains. Thus, a thermometer placed in the water does not register even approximately the actual temperature applied to the patient's skin. We have therefore used flat-bottomed silver tubes with a diameter of 1-25 cm. These tubes were filled with broken ice, or with water at the temperature desired, and contained a thermometer. They were never warmed or cooled from without. When used for testing sensibility to heat, several tubes ranged in a wooden stand were filled with water at temperatures considerably higher than those we wished to use for testing; from these, a tube was selected as soon as it had sunk to the temperature required. These silver tubes lose their heat so rapidly that it is impossible to use the same one for more than a short series of tests.
Sometimes, when testing large areas of sensibility to heat or to cold produced by lesions of the spinal cord, we have employed large copper tubes of 4 cm. in diameter. These retain their temperature much longer than the smaller silver tubes, and are particularly useful when we are concerned mainly with the existence of sensibility either to heat or to cold rather than with the exact degree of thermal stimulation; they also form an excellent means of evoking a thalamic over-reaction to temperature stimuli.
It is well to remember, when testing the scalp, that the hair insulates the skin, so that both heat and cold pass through with difficulty, and the results are liable to be unsatisfactory.
Few difficulties attend the testing of sensibility to the more extreme degrees of heat and cold. But occasionally, when the affected parts are sensitive to painful stimuli but not to heat, a tube of 50° C. or above may be said to be hot solely on account of the peculiar pain produced. This is particularly the case when sensation is returning after division of a peripheral nerve, or with lesions of the spinal cord which destroy sensibility to heat but not to painful stimuli. During the experiments on my arm, when tested with these temperatures, I frequently said, "Any ordinary patient would have called such stimuli hot, because the pain produced is of a kind associated in daily life with the action of hot bodies only. Further, a patient is told to say if he feels heat, cold, or a touch. Given, then, that he knows his thermal sensibility is being tested, he would certainly call the sensation I experience 'hot.'"
Occasionally contact with a neutral tube would cause an indeterminate and somewhat tingling sensation over the affected area; this was frequently
