HUNGER AND THIRST. 167 



impression to the encephalon. The nerve which would naturally be sup- 

 posed to have this office is the pneumogastric ; but notwithstanding certain 

 observations to the contrary, it has been shown that section of both of these 

 nerves by no means abolishes the desire for food. Longet has observed that 

 dogs eat, apparently with satisfaction, after section of the glosso-pharyngeal 

 and lingual nerves. This observer is of the opinion that the sensation of 

 hunger is conveyed to the brain through the sympathetic system. Although 

 there are various considerations which render this somewhat probable, it is 

 not apparent how it could be demonstrated experimentally. It is undoubt- 

 edly the sympathetic system of nerves which presides specially over nutrition ; 

 and hunger, which depends upon deficiency of nutrition, is certainly not con- 

 veyed to the brain by any of the cerebro-spinal nerves. 



Thirst is the peculiar sensation which leads to the ingestion of water. 

 In its moderate development, this is usually an indefinite feeling, accom- 

 panied by more or less sense of dryness and heat of the throat and fauces, 

 and sometimes, after the ingestion of a quantity of very dry food, by a 

 sensation referred to the stomach. When the sensation of thirst has become 

 intense, the immediate satisfaction which follows the ingestion of a liquid, 

 particularly water, is very great. Thirst is very much under the influence of 

 habit ; some persons experiencing a desire to take liquids only two or three 

 times daily, while others do so much more frequently. The sensation is also 

 sensibly influenced by the condition of the atmosphere as regards moisture, 

 by exercise and by other conditions which influence the discharge of 

 water from the body, particularly by the skin. A copious loss of blood is 

 always followed by great thirst. This is frequently noticed in the inferior 

 animals. After an operation involving haemorrhage, they nearly always drink 

 with avidity as soon as released. In diseases which are characterized by 

 increased discharge of liquids, thirst is generally excessive. 



The demand on the part of the system for water is much more imperative 

 than for solids ; in this respect being second only to the demand for oxygen. 

 Animals will live much longer when deprived of solid food but allowed to 

 drink freely than if deprived of both food and drink. A man, supplied with 

 dry food but deprived of water, will not survive more than a few days. Water 

 is necessary to the processes of nutrition, and acts, moreover, as a solvent in 

 removing from the system the products of disassimilation. 



After deprivation of water for a considerable time, the intense thirst be- 

 comes most distressing. The dryness and heat of the throat and fauces are 

 increased and accompanied with a sense of constriction. A general febrile 

 condition supervenes, the blood is diminished in quantity and becomes thick- 

 ened, the urine is scanty and scalding, and there seems to be a condition of 

 the principal viscera approaching inflammation. Death takes place in a few 

 days, generally preceded by delirium. 



The sensation of thirst is instinctively referred to the mouth, throat and 

 fauces ; but it is not necessarily appeased by the passage of water over these 

 parts, and it may be effectually relieved by the introduction of water into the 

 system by other channels, as by injecting it into the veins. Bernard has 



