660 A TEXTBOOK OF PHYSIOLOGY 



pencil-point is connected with this mechanism, and persists when 

 the cutaneous nerves have been cut. The nerves connected with 

 this sense pass into the cord by the posterior roots, and have their 

 cell-stations in the posterior root ganglion. The distribution of the 

 fibres is referred to later (see p. 673). 



The sense of "balance" is due to a co-ordination of impulses 

 received from the eyes, the semicircular canals, and the organs of ctita- 

 neous and kinsesthetic sensibility. The aiiman is particulaily depen- 

 dent upon his sense of vision. He also derives much information as 

 to the position of his machine by cutaneous sensations from his seat 

 and from the play of the wind on his cheeks. The <; feel " of the joy- 

 stick in which kinsesthetic sensations play a part is also important, as 

 we'll as the impulses received from his vestibular apparatus. 



The Entero-ceptive Mechanism. The entero-ceptive mechanism is 

 associated with the sensations arising from the alimentary tract from 

 the beginning of the gullet down to the rectum. Various forms of 

 receptor mechanisms have been described, varying in nature from 

 free expansions of dendritic nerve-endings to the elaborate Pacinian 

 corpuscle. The total number of afferent fibres to the viscera is small 

 not more than those contained in a single posterior root . By means 

 of this mechanism, the orderly sequence of the movements of the 

 digestive tract is insured, usually without involving consciousness of 

 the process. 



The different parts of the tract vary to different forms of stimula- 

 tion in their power to provoke conscious sensitivity. From the 

 beginning of the oesophagus to the junction of the rectum the 

 alimentary tract is insensitive to tactile stimuli. Heat and cold do 

 not excite the mucous membrane of the stomach; the colon is almost 

 insensitive; the gullet and anal canal, on the other hand, are sensitive 

 to thermal stimulation. 



Chemical stimuli also vary in effect. Alcohol produces a feeling 

 of warmth in all parts, whereas glycerine has 9, localized stimulatory 

 effect upon the anal canal. The mucous membrane of the oesophagus 

 and stomach is insensitive to stimulation with weak hydrochloric acid. 



The sensation of thirst is due to changes in the mouth, throat. 

 and stomach. It is generally brought about by a drying of the 

 mucous membrane of the throat after the inhalation of dry or dusty 

 air, or the ingestion of salt or dry food. When water is long with- 

 held, it is possible the terrible sensation of thirst also arises from an 

 altered condition of the blood. 



The sensation of hunger is evoked by the contractions of the con- 

 tracted empty stomach. It may be satisfied to a certain extent by 

 swallowing the saliva to induce relaxation, hence the efficacy of 

 chewing tobacco. Normally, food induces such a relaxation. It 

 may, however, be induced by swallowing any solid material. Thus. 

 the natives on the Orinoco appeased their hunger in the time of 

 need by eating baked earth. The distension of the stomach thus 

 induced satisfies hunger by stopping its movements. The sensation 

 of fulness after a large meal is due to dilatation of the stomach. 



