THE SENSES 853 



that most mucous membranes are in comparison with the skin but 

 slightly sensitive to changes of temperature ; some, as the mucous 

 membrane of the greater portion of the alimentary canal, seem to be 

 entirely devoid of nerves of temperature. Only towards its ends in 

 the mouth, pharynx and rectum, and to some extent in the stomach, 

 does a blunted sensibility appear. The uterus, too, is quite insensible 

 to heat; and hot liquids may be injected into its cavity at a temperature 

 higher than that which can be borne by the hand, without causing 

 inconvenience a fact which finds its application in the practice 

 of gynaecology and obstetrics. It is, indeed, obvious that in the 

 greater number of the internal organs the conditions necessary for 

 stimulation of temperature nerves, even if such were present, could 

 hardly ever exist. 



It has already been mentioned that changes of external temperature 

 exert a remarkable influence on the intensity of metabolism (p. 510), 

 and it has been supposed that this is brought about by afferent 

 impulses travelling up the cutaneous nerves. We have also seen 

 that for certain kinds of stimuli the excitability of nerve-fibres is 

 increased by cooling (p. 594). It is possible that this is the case for 

 the fibres in the skin which are concerned in the regulation of the 

 production of heat, and it has been suggested that this fact may have 

 a bearing on the reflex regulation of temperature (Lorrain Smith). 



The Muscular Sense. 



Voluntary muscular movements are accompanied with a 

 peculiar sensation of effort, graduated according to the 

 strength of the contraction, and affording data from which 

 a judgment as to its amount and direction may be formed. 

 To these sensations the name of the muscular sense has been 

 given. 



Some writers have supposed that the muscular sense 

 does not depend upon afferent impulses at all, but that 

 the nervous centres from which the voluntary impulses 

 depart, take cognizance, retain a record, so to speak, of the 

 quantity of outgoing nervous force; that the effort which 

 we feel in lifting a heavy weight is an effort of the cells of 

 the motor centres from which the innervation of the groups 

 of muscles takes origin, and not of the muscles themselves. 



But although this feeling of central effort or outflow (we 

 can hardly say of central fatigue) may play a part in the 

 muscular sense, it cannot be doubted that the brain is kept 

 in touch with the contracting muscle by impulses of various 

 kinds which reach it by different afferent channels. 



