All of our dogs used in this work were provided with a gastric fistula for the introduction of the stomach balloon. The esophagus was left intact and the esophageal balloon introduced through the mouth. The dog is of special interest in this connection, because the dog's esophagus is composed of striated musculature throughout its whole length. The myenteric plexus is probably lacking. It is needless to say that all the dogs were subjected to preliminary training in the way of getting used to the balloons in stomach and esophagus. As a matter of fact all the dogs were accustomed to these procedures, having been used for other lines of work on the hunger mechanism, so that they would lie quietly and comfortably in the lap of an attendant during the tests. It is absolutely essential that the dogs be quiet, if possible sleeping, during these tests, for restlessness increases the disturbance of the esophagus even more than of the stomach. *
The presence of the inflated balloon in any region of the esophagus caused rapid local contractions, more rapicl than those of the human esophagus, alternating with an occasional peristalsis, also of local origin, and occasionally more prolonged tetanic contractions. These tetanic contractions usually last for only half a minute to a minute. In one case it lasted 15 minutes. Tetanic spasms lasting up to 5 minutes are not uncommon. These contractions have no relation to the hunger contractions of the empty stomach, as they may appear during a hunger period as well as during gastric quiescence. They depend on the local stimulation of the balloon in the esophagus and are, therefore, more marked the greater the tension in the balloon. They are also more marked when the dogs are excited, evidently owing to increased reflex excitability of the medullary centers and some increase in the tonus of the esophagus. The disturbance is greatest when the balloon is first introduced, but it may keep up for hours even when the pressure in the balloon is only 2 to 3 cm. of bromoform or chloroform. A single swallowing act may induce these local contractions, lasting for many minutes, in a quiescent esophagus. The same thing has been observed in man. It is probably due to increased reflex excitability of the medulla and to increased tonus of the esophagus, as the latter is equivalent to increased tension in the balloon, and therefore increased strength of the local mechanical stimulation.
When the esophagus contracts in tetanus on the balloon, additional disturbing factors appear. Evidently this kind of contraction causes the same sensation in man and dog, that is, the feeling of something stuck in the throat, for when these contractions are present the dogs become restless and sometimes swallow repeatedly.
The gastric hunger contractions are not accompanied by contractions or changes of tonus in any part of the esophagus. If the gastric hunger contractions are very intense, so that they cause the dog to moan or swallow, contractions may appear in the quiescent esophagus, but this is obviously due to the general disturbance of the animal. There is no synchrony between the gastric and the esophageal contractions. If the dog lies quietly or is sleeping, a strong gastric hunger period may run its course without the least evidence of esophageal contractions.
The strong and prolonged local contractions of the esophagus cause inhibition of the gastric tonus and the gastric hunger contractions, while the rapid esophageal contractions seem to have no effect on the stomach. This is evidently a special instance of the "law of the intestine," that is, inhibition caudal or aboral to a region in the state of contraction. Other factors, may also be concerned in this inhibition of the stomach. The tetany of the esophagus causes some distress, and this may lead to inhibitory action via the splanchnic nerves. The gastric inhibition ordinarily lasts as long as the tetanus in the esophagus.
Seeing and smelling meat on the part of a hungry dog almost invariably led to contractions of the otherwise quiescent esophagus. Ordinarily the contractions are of the local rapid type, and the disturbance of the esophagus lasts only for 10 to 30 seconds. But occasionally the sight or smell of food may send the esophagus into complete tetanus lasting for several minutes. Both types of esophageal contractions are accompanied by inhibition of the stomach. A single act of swallowing, as we have seen, may produce the same disturbance in the part of the esophagus where the balloon is located, and the dog frequently swallows or at least elevates the larynx at the sight of the food. But these contractions on the sight of food also appear when the dog does not swallow. The phenomenon is probably to be explained as follows: The sight or smell of food on the part of the hungry dog causes temporary increase in neuro-muscular tonus, including the tonus of the esophagus. The increase in esophagus tonus causes greater pressure on the balloon, and in consequence stronger mechanical stimulation of the afferent nerves in the esophagus, and hence the rapid or tetanic contractions. It is therefore probable that the actual contractions of the esophagus on seeing food are artifacts, so to speak, due to the presence of the balloon.