A third series of experiments was carried out on the author and on one assistant (Mr. B.). Three rubber tubes of suitable size were placed one inside the other so as to have three walls of rubber and two air spaces between the liquid in the inside tube and the esophageal mucosa, for the purpose of retarding heat conduction. The inside tube had a diameter of 3 mm. The subject was blindfolded and 50 c.c. of water at 50° C. were injected into the stomach. A sensation of heat was noted about 10 to 15 seconds after the injection, and about 60 seconds before enough conduction had taken place to cause the tube to feel warm in the mouth and throat. When the same amount of water at 10° C. was injected, a definite cold sensation was felt from 5 to 10 seconds after the injection had commenced and about 60 seconds before the tube began to feel cold to the fingers or to the throat.
Both of us experienced hot and cold sensations and never made the mistake of confusing the heat sensation with the cold sensation. Confirming Neumann and others, we found the cold sensation to be more distinct and better localized than the heat sensation.
This certainly does not seem to indicate that the stomach mucosa is devoid of temperature sensations, nor that this sensation is projected from the esophagus, as has been suggested by a number of investigators. However, there is a possibility that some of the water might reach the wall of the lower end of the esophagus by being forced up along the tube. In view of the small amount of water necessary to evoke the hot and cold sensations and in view of the short latent period mentioned above, this seems very unlikely.
To make sure that the sensation did not arise as a result of the stimulation of the esophagus, as well as to prove that the sensation is not due to conduction to the skin of the abdomen, or to reflex vascular changes in the skin of the abdomen, a fourth series of experiments was performed. For these experiments a rubber tube i cm. in diameter was used. Inside this large tube we placed two smaller tubes, side by side; one of these was 5 mm. in diameter and the other was 3 mm. The smaller of these two tubes was not pulled clear through the outside tube, but only so far that its lower end was about an inch above the lower end of the outside tube. The 5 mm. tube was pulled through the outside tube so that it extended about 10 cm. below the lower end of it. A small glass tube 5 mm. long was now forced up into this longer tube as far as to the lower end of the outside tube. The lower end of the outside tube was then tied securely about the longest tube at this point. This apparatus was now swallowed far enough down, so that the lower end of the outside tube reached just through the cardiac orifice. The longest tube, the end of which of course was open, therefore extended far down into the stomach. The smallest, whose lower end was within an inch of where the outside tube was tied, that is, within 2 to 3 cm. of the cardia, was now connected with a pressure bottle. It had previously been found that hot water, in passing through a glass tubing and connections of the pressure bottle, lost approximately 8° C, and that water placed in the bottle at 10° C. was raised to 120 C. in passing through. Water at 58° C. was now placed in the pressure bottle and permitted to flow down through the smallest inside tube. It naturally was forced up again between and around the two inside tubes. By so doing the heat was conducted through the wall of the outside tube and stimulated the heat nerve-endings in the esophagus, so that a definite heat sensation was felt along its whole course, as well as in the mouth and pharynx. The striking feature about this, however, was that, although the walls of the lower end of the outside tube naturally became hot before those of the upper, the heat sensation was first felt in the mouth and throat and then gradually traveled down the whole length of the esophagus. At all times, however, the heat sensation was more intense in the throat and in the upper part of the esophagus than in its lower end. While the walls of the esophagus were thus being stimulated, 50 c.c. of water at 50° to 55° C. was injected into the stomach through the longest inside tube which opened into the stomach, and after a latent period of about 10 seconds, a spreading heat sensation was felt lower down than that resulting from the stimulation of the esophagus. The sensation also seemed better localized than that from the esophagus, so that it was not very difficult to keep the two sensations apart in consciousness. The reason for the better localization might, of course, be due to the fact that more nerve-endings were stimulated in the stomach mucosa, as the water came into direct contact with it, than in the case of the esophagus, where probably the rubber tube did not come into contact with the mucosa at all points.
When water at 10° C. was placed in the pressure bottle-the outside tube stimulating the esophagus thus being filled by water at 120 C. and 50 c.c. of water at 120 C. was injected into the stomach, a distinct, spreading, cold sensation was felt lower than that resulting from the stimulation of the esophagus. There was no difficulty in recognizing two separate and distinct cold sensations, one coming from the esophagus and the other from the stomach.
While the esophagus was being stimulated by water at 120 C. in this tube system, a water bottle which was filled with water at 120 C. was placed on the skin of the abdomen. The two cold sensations resulting could easily be kept apart in consciousness. Next, 50 c.c. of water at the same temperature were injected into the stomach, and a third, distinct, spreading, cold sensation was felt in the stomach. This experiment was repeated several times with hot and cold water, as indicated above, and with the same results.
If the temperature sensation felt on introducing hot or cold liquids into the stomach is due to heat conduction through to the skin of the abdomen, or to reflex vascular changes in the skin of the abdomen, or is projected from the esophagus, how is it possible that one can distinguish in consciousness the sensations coming from the esophagus', the stomach, and the skin, at the same time, when each is being stimulated by water at the same temperature, as shown above ?
To make sure that the sensation was not due to conduction through the abdominal wall to the skin of the abdomen, the bulb of a thermometer was pressed close to the skin over the stomach and covered with absorbent cotton. After it had become stable, 200 c.c. of water at 10° C. were injected into the stomach and the mercury in the thermometer watched for a period of 5 minutes. This was repeated several times. In each case, after a latent period of 5 minutes, the mercury had fallen only a small fraction of a degree. It is therefore evident that the cold sensation which one feels after a latent period of from 5 to 10 seconds, after the injection of 20 c.c. of cold water into the stomach, is not due to conduction through the abdominal wall and stimulation of the nerve-endings in the skin.
Boring has shown that 500 c.c. of water at o° C. in the stomach lowers the skin temperature only 0.050 C. It is therefore out of the question that 25 c.c. of water in the stomach at 250 to 300 C. stimulate the temperature end organs in the skin. Moreover, the adequate temperature stimulus in the stomach cavity is less than 50 above and 10° below the body temperature.
We agree with Hertz that when one swallows a mouthful of ice water, the most intense cold sensation seems to come from the lower end of the esophagus, and that this is undoubtedly due to the fact that the water accumulates there and is detained for a shorter or longer period before the cardiac orifice opens so that it can flow into the stomach. He further states that the cold sensation disappears after the water has entered the stomach. This we cannot corroborate. We invariably feel a vague, spreading, cold sensation after the water has entered the stomach, which disappears after a few seconds. Even if it were the case that no cold sensation could be felt after the water had passed into the stomach, this could not prove that the stomach mucosa is devoid of heat and cold nerve-endings. In the first place, a mouthful of cold water in passing down into the stomach is warmed sufficiently to raise its temperature several degrees. On this account the nerve-endings in the mucosa of the stomach are not stimulated as intensely as are those in the esophagus. The empty stomach contains 10 to 50 c.c. of fluid at 380 C. which rapidly mixes with and thus raises the temperature of the swallowed water. It is also well known that stronger impulses, reaching the central nervous system from one part, tend to suppress in consciousness weaker impulses of the same nature, reaching it from some other part, so that it is difficult to separate the two sensations in consciousness.
From our experiments we conclude: (1) The stomach mucosa is endowed with heat and cold nerve-endings. (2) These fibers are, as Head suggests, of the protopathic type; that is, they are not stimulated by slight temperature changes, or if they are, the impulses do not affect consciousness. (3) They are more abundant, or more readily stimulated in the throat and esophagus than in the stomach.