THE SENSES 573 



Muscle-sense has been referred to in the chapter on the 

 Muscular System (p. 390), Nervous System (p. 470), and will be 

 again met with in dealing with Locomotion. 



Hunger. 



The sensation of hunger is referred to the stomach. It has 

 been supposed that it is excited by the approximation of the 

 wahs of the organ, but animals may exhibit hunger at a time 

 when the stomach contains food — for instance, the horse and the 

 rabbit. Nothing is known of the nervous mechanism connected 

 with this condition. It is one of the senses under the control 

 of a lowly organised part of the brain, for a dog without its 

 cerebrum shows the usual signs, though obviously it cannot be 

 associated with any conscious knowledge of want. The length 

 of time animals will withstand starvation is dealt with at p. 363. 

 A horse in good condition has been known to live thirty days 

 provided water be given ; but an animal in poor condition to 

 start with has died in half that time. The remarkable period 

 during which sheep have lived without food has been mentioned 

 at p. 363, and a similar fact has been recorded in the pig ;* the 

 animal was buried for sixty days in a landslip, and was recovered 

 alive, having lost in that time 120 pounds in weight. It is not 

 known why eating gives practically immediate relief to all the 

 symptoms of hunger, for it is obvious that the food ingested can 

 be of no use until absorption has commenced. 



Thirst. 



Very little is known of the subject of thirst — not even why the 

 sensation is referred to the pharynx. It has been supposed that 

 the glossopharyngeal nerve has special nerve-endings in the 

 pharynx, which are stimulated when the water content of the 

 body falls below a certain point ; for if the palate be moistened 

 thirst is allayed, while filling the stomach with water through a 

 fistula does not at once control the desire for fluid. If a horse 

 be ' sham watered ' — viz., with a fistula in the oesophagus — he 

 leaves off drinking, after a few swallows, as if satisfied, and then 

 starts again as the pharynx gets dry (Colin). 



There is a constant loss of water occurring in the body-fluid 

 through respiration and the various secretions, such as sweat, 

 urine, milk, and digestive juices. Some of the fluid of the latter 

 may be reabsorbed ; especially is this likely to occur in such 

 bulky secretions as saliva in the herbivora. The requirements 

 of an animal for water are constant, though varying with the 



* Martell, Transactions of the Linnean Society^ vol. ix. 



