332 OF DIGESTION AND NUTRITIVE ABSORPTION. 



known of the effect produced upon it by nervous impressions, which have a 

 peculiar influence upon the capillary circulation. It also corresponds exactly 

 with what we know of the influence of the nervous system, and of mental 

 impressions, upon other secretions ($ 426). 



442. The sense of Hunger, like other sensations, may not be taken cogni- 

 zance of by the mind, if its attention be strongly directed towards other objects ; 

 of this fact, almost every one engaged in active occupations, whether mental 

 or bodily, is occasionally conscious. The nocturnal student, who takes a light 

 and early evening meal, and, after devoting himself to his pursuits for several 

 hours uninterruptedly, retires to rest with a wearied head and an empty 

 stomach, but without the least sensation of hunger, is frequently prevented 

 from sleeping by an indescribable feeling of restlessness and deficiency; and 

 the introduction of a small quantity of food into the stomach will almost instan- 

 taneously allay this, and procure comfortable rest. Many persons, again, who 

 desire to take active exercise before breakfast, are prevented from doing so 

 by the lassitude and even faintness which it induces, the bodily exercise 

 increasing the demand for food, whilst it draws off the attention from the sen- 

 sation of hunger. The Author may be excused for mentioning the following 

 circumstance, which some years ago occurred to himself; and which seems to 

 him a good illustration of the principle, that the sense of hunger originates 

 in the condition of the general system, and that its manifestation through a 

 peculiar action in the stomach, is to be regarded as a secondary phenomenon, 

 . adapted, under ordinary circumstances, to arouse the mind to the actions 

 necessary for the supply of the physical wants, but capable of being over- 

 looked, if the attention of the mind be otherwise directed. He was walking 

 alone through a beautiful country, and with much to occupy his mind; and, 

 having expected to meet with some opportunity of obtaining refreshment on 

 his road, he had taken no food since his breakfast. This expectation, how- 

 ever, was not fulfilled ; but, as he felt no hunger, he thought little of the dis- 

 appointment. It was evening before he approached the place of his destina- 

 tion, after having walked about twenty miles, resting frequently by the way ; 

 and he then began to feel a peculiar lassitude, different from ordinary fatigue, 

 which rapidly increased, so that during the last mile he could scarcely support 

 himself. The " stimulus of necessity," however, kept him up ; but on arriving 

 at his temporary home, he immediately fainted. It is obvious that, in this 

 case, the occupation of the mind on the objects around, and on its own thoughts, 

 had prevented the usual warning of hunger from being perceived; and the 

 effect which succeeded was exactly what was to be anticipated, from the 

 exhaustion of the supply of food occasioned by the active and prolonged exer- 

 tion. 



443. The conditions of the sense of Thirst appear to be very analogous to 

 those of hunger. This sense is not referred, however, to the stomach, but to 

 the fauces. It is generally considered that it immediately results from an 

 impression on the nerves of the stomach ; since, if liquids are introduced into 

 the stomach through an oesophagus tube, they are just as effectual in allaying 

 thirst, as if they are swallowed in the ordinary manner. It may, however, be 

 doubted whether the sense of thirst is not even more immediately connected 

 with the state of the general system than that of hunger; for the immediate 

 relief afforded by the introduction of liquid into the stomach is fully accounted 

 for, by the instantaneous absorption of the fluid into the veins, which is known 

 to take place, when there is a demand for it, not only from Dr. Beaumont's 

 observations, but from many experiments made with reference to this particu- 

 lar question. This demand is increased with almost equal rapidity, by any 

 excess in the amount of the fluid excretions ; and it may be satisfied without 



