CUTANEOUS AND INTERNAL SENSATIONS 1043 



lated, and the sensations of lukewarm and then of warm are experi- 

 enced. When the temperature of the water reaches 45 C, the 

 quality of the sensation changes to ' hot.' At a still higher tempera- 

 ture the sensation becomes painful or burning. The most probable 

 explanation of these facts is mentioned below (p. 1044). 



It is not only of physiological interest, but of practical importance, 

 that most mucous membranes are in comparison with the skin but 

 slightly sensitive to changes of temperature. Only towards the ends 

 of the alimentary canal, in the mouth, pharynx, oesophagus, and anal 

 canal, is it possible to elicit warmth or cold sensations. There is some 

 difference of opinion whether a blunted sensibility appears in the 

 stomach also. The uterus, too, is quite insensible to moderate 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 inconveni- 

 ence 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 tem- 

 perature 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. 668), 

 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. 758). 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) . 



Pain Sensations. While the cold and the warmth spots are irregu- 

 larly distributed over the skin in more or less compact groups, and 

 the touch sensations are intimately associated with the hair follicles, 

 the pain spots are more uniformly spread, and at the same time set 

 closer together. In parts of the body where but one of these 

 elementary forms of general sensibility is present, as in the central 

 parts of the cornea and in the dentine and pulp of the teeth, it is 

 always pain. 



In certain situations pain and temperature sensibility are found 

 together, but not touch e.g., at the margin of the cornea and on 

 the conjunctiva. 



In general, the skin is far more sensitive to pain than the deeper 

 structures. The most painful part of an operation is generally the 

 stitching of the wound. The cutting of healthy muscle causes no 

 pain. In an operation in which an artificial connection was estab- 

 lished between the stomach and the small intestine (gastro-enter- 

 ostomy), and in which no anaesthetic was administered, the only 

 pain of which the patient complained was produced by the incision 

 in the skin (Senn). This, however, does not prove that the 

 abdominal viscera are devoid of pain nerves, for it has been shown 

 in animals that exposure of the intestines, etc., as in laparotomy, 

 leads to a rapid depression (exhaustion ?) of the sensibility for pain 



