The experiments on dogs were made on pups delivered by Caesarian section 8 to 14 days before term, on pups 5 weeks to 5 months of age, on young adults, and on old adults. In all cases care was taken to choose only dogs in good condition and perfect health, as far as could be judged by actions and external appearances. Except in the case of the prematurely born pups, the observations were made 24 hours after feeding, in order to assure a completely empty stomach. The animals were given at least one day or more of rest after each experiment, so as to be in a perfectly normal condition when used again. During the taking of the records they were held in the lap, apparently without any appreciable discomfort, for they nearly always slept through a large part of the experiment. Ten different series of records of the hunger movements of the empty stomach were obtained from each dog on ten separate days, the continuous experimental periods ranging from 2 1/2 to 4 1/2 hours, respectively.

The results summarized in the accompanying table were computed from the tracings, and the figures in each case represent as nearly as possible the true time of activity and rest of the empty stomach, everything in the records of a doubtful character, or of an abnormal nature caused by disturbing influences, being eliminated.

Gastric Hunger Contractions In Dogs Of Different Ages

As regards variation of stomach movements between dogs of different ages, the chief and practically the only constant difference was found in the length of the periods of contraction and the periods of quiescence. In all cases the periods of quiescence are the longest in old dogs, varying from 1 1/6 to 4 1/6 hours, and rapidly decreasing in length proportionately to age to 2 1/2 to 3 4/10 minutes in pups a few days to five or six weeks old. Conversely the periods of contraction are the longest in the young dogs-for instance, in the very young pup the recorded periods run from 4J to 5^ hours-and they rapidly decrease in length proportionately to age-in the old dogs from 30 minutes to 2 hours-thus showing that the stomach's activity is in direct proportion to the age of the animal (Patterson).

The rapidity of the strong hunger contractions during the active periods appears on the whole to be greater in young animals than in old. The tonus of the stomach and also the strength of the contractions in young animals may be slightly higher, but: they are subject to great variations. The decrease in the activity of the stomach as the animal approaches senility is probably an explanation, in part at least, of the more chronic gastric disturbances in the aged.

To summarize: in healthy dogs the hunger contractions of the empty stomach decrease with age. This decrease appears to some extent in the tonus and in the rapidity of the hunger contractions, but is particularly marked in the duration of the periods of hunger activity and the intervening periods of quiescence of the stomach. On the whole, the decrease in the gastric hunger activity is proportional to the advance in age. In very young dogs the hunger contractions of the empty stomach are practically continuous.

Two factors are probably involved in this age variation in the gastric hunger contractions, namely, (1) the actual age (reduced metabolism) of the motor tissue in the stomach itself, and (2) a lowering of metabolism in the entire body, which may imply a smaller quantity of chemical stimuli to the gastric hunger mechanism in the circulation. Patterson attempted to determine the relative importance of these two factors by studying the influence of prolonged starvation on the gastric hunger contractions in very young and in very old dogs. If the rate of general body metabolism, that is, starvation metabolism, is the main factor, we should expect that in prolonged starvation the gastric hunger contractions of old dogs would approach the vigor of the normal hunger contractions of young dogs. This is not the case. It is true that starvation causes some increase in the vigor of the stomach tonus and contractions in the old dogs. Starvation metabolism of the body in general is therefore a factor. But the actual age of the stomach tissues is the most important element. How actual age of the gastric motor mechanism reduces the vigor and duration of the hunger contractions cannot be answered until we know what constitutes the physiological aging of the tissues.