846 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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." 1 



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 equilibrium, 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 eyes closed, a writing point attached to the head will show that 

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

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

 sibility, as in locomotor ataxy, cannot stand \vith eyes closed, and his move- 

 ments, even when sight is employed, are exaggerated and unnatural. Attention 

 has previously been called to the fact that air- waves, irrespective of those 

 producing sound-sensations, exert an influence upon the tympanic membrane 

 by which we are capable of appreciating the presence and, to some extent, the 

 physical character of objects. Whether this sensation involves the nerves of 

 touch, those of common sensibility, or those distributed to the internal ear, is 

 uncertain. 



In the absence of any of these sensations the loss may be made up by more 

 perfect development of others. Ordinarily, the sensory information from all 

 these sources, when compared in consciousness, harmonizes and gives rise to 

 a concrete idea of position. Frequently, however, one of the sources of sense- 

 impression suddenly fails us or its testimony conflicts with that of other sense 

 1 Ewald : Diseases of the Stomach, p. 397. 



