404 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



to two cases in which, while the tactile sense was preserved, muscular sensation 

 was lost, and an object could be held in the grasp only while the eyes were 

 turned upon it. 



Hunger and Thirst. — Hunger and thirst are peculiar sensations which 

 depend partly on local and partly on general causes. Diminution in the bulk 

 of water and of circulating aliment in the body no doubt causes excitement 

 of sensory nerves on which depend the feelings of thirst and hunger, but in 

 ordinary life these feelings are dependent on the physical condition of certain 

 mucous surfaces. Any circumstance which causes drying of the lining mem- 

 brane of the mouth provokes thirst, and some condition of the empty stomach 

 arouses hunger. Thirst may be assuaged by introducing water directly into 

 the stomach through a gastric fistula, though to effect the purpose a larger 

 quantity must be employed in this way than by the mouth. Hunger in a 

 somewhat similar manner may be appeased by rectal alimentation. It seems 

 probable, however, that these sensations as usually felt are the result of a 

 sort of habit, depending on the physiological condition of the secreting and 

 absorbing mechanisms of the alimentary canal. 



Clinical observation has shown that " bulimia," or voracious appetite, is 

 frequently a result of disease in certain parts of the central nervous system. 

 We are therefore justified in speaking of a " hunger-centre." ' 



E. The Equilibrium of the Body; the Function of the 

 Semicircular Canals. 



The term equilibrium, as applied to the condition of the body, whether at 

 rest or in motion, indicates a state in which all the skeletal muscles are under 

 control of nerve-centres, so that they combine, when required, to resist the 

 effect of gravity or to execute some co-ordinated motion. The preservation 

 of equilibrium is manifestly of fundamental importance in animal life, and we 

 find, accordingly, several mechanisms sharing in this function. That the motor 

 co-ordinating centres may act properly, they must receive sensory impres- 

 sions conveying information of the relative position of the body at any given 

 moment. The sum-total of these sensations may be characterized as the sense 

 of equUibrivm, and it is probably not going too far to assume that every known 

 sensation contributes to this fund of information. Thus, in ordinary life the 

 position of objects is commonly determined by the sense of sight: when one 

 tries to walk while looking through a prism, objects are not properly localized 

 by vision, and improper co-ordination results. The contact of the soles of the 

 feet with the ground, and that of the surface of the body with various objects, 

 are common sources of information as to our relation with the environment. 

 Standing upright, and still more when in motion, the muscular sense is active 

 in appreciating the tension, active or passive, of the muscles. In the erect 

 position, with eves closed, a writing point attached to the head will show that 

 the body sways in a peculiar manner indicating successive contraction of differ- 

 ent groups of muscles; and a person with failure of muscular and tactile sen- 

 1 Ewald : Disease a of the Stomach, p. 397. 



