THE SENSES 



973 



number of the internal organs the conditions necessary for stimula- 

 tion of temperature nerves, even if such were present, could hardly 

 ever exist. 



It has already been mentioned that changes of external tempera- 

 ture exert a remarkable influence on the intensity of metabolism 

 (p. 590), 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. 681). 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. 



While the cold and the warmth spots are irregularly 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 established between the stomach and the small 

 intestine (gastro-enterostomy), 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 (Kast and Meltzer). 

 In the intact animal and human being painful impressions can 

 unquestionably be excited in the viscera by adequate stimuli 

 (p. 799). Thus, the spasmodic contraction of the intestines and 

 stomach causes the intense pain of colic and gastralgia. Labour 

 is an example of a strictly physiological function which is the 

 occasion of severe pain. Tissues normally insensible, or, rather, 

 but slightly sensible, to pain may become acutely painful when 

 inflamed. 



The question has been raised whether the sensation of pain 

 can be caused by excessive stimulation of the nerves of common 

 tactile sensibility, or of the nerves that subserve the sensations 



