692 PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



VI. HUNGER AND THIKST. 



The sensations which lead to the prehension of solid and liquid 

 foods are known as hunger and thirst. It was previously supposed that 

 hunger was a local sensation which was produced by the absence of food 

 in the stomach. Evidently this is a mistaken idea, else would the rumi- 

 nants never experience this sensation, for in them the stomach is never 

 free from food, even when death occurs from starvation. The appear- 

 ance of hunger coincides with absorption of the matters digested at the 

 previous meal. Although the sensation is apparently referred to the 

 abdomen, it cannot be regarded as a localized sensation, but rather as a 

 peculiar modification of the general system similar to that produced in 

 dyspnoea ; nor, indeed, is the stomach even the starting point of hunger, 

 for the pneumogastrics, the sensory nerves of this organ, may be divided, 

 and if the animals be deprived of food the clearest evidences of hunger 

 are, nevertheless, capable of detection. In spite of this fact, the sensation 

 of hunger is, nevertheless, to a certain extent dependent upon the con- 

 dition of the stomach, as is indicated by the temporary relief of hunger 

 which follows introduction into the stomach of matter which is not in 

 the slightest respect nutritious. So, again, even when the stomach is 

 filled with food, if through any disease digestion or absorption or the 

 passage of food into the small intestine are interfered with the hunger 

 may, nevertheless, be intensely felt ; and, again, even after a hearty meal 

 digestion may be complete, and the stomach empty for some time before 

 the sensation of hunger appears. 



In the case of thirst the state of affairs is somewhat similar, except 

 that there the sensation is more distinctly localized in the fauces and 

 may be relieved by the application of moisture to that part. When an 

 animal is deprived of liquid, the blood, from the continued formation of 

 secretions and excretions, rapidly loses its normal percentage of water. 

 The sensation of thirst is evidently due to the irritation of the sensory 

 nerves of the mucous membrane of the pharynx produced by the drying 

 of the mucous membrane, and while it may be relieved, as already 

 mentioned, by moistening this part, the arrest. of thirst is only temporary. 

 But, on the other hand, the thirst may be permanently relieved by the 

 injection of water into the veins and even by enemata of water, and 

 while thirst, therefore, has a local expression, like hunger, it represents 

 the needs of the economy for water ; thirst may thus be abolished, even 

 although no water enter the mouth. Every cause, therefore, which 

 diminishes the proportion of fluid in the blood, whether intense heat or 

 exercise which favor cutaneous and pulmonary evaporation, dropsies, 

 abundant hemorrhages, or diabetes, all lead to thirst ; so, also, salts 

 occasion thirst by withdrawing water from the blood. 



