52 PHYSIOLOGY CHAP. 



proof that it exists in the lower animals. By a minute analysis 

 of the motor reactions of Lumbricus, Normann has proved that 

 they cannot have the significance of expressions of pain, because 

 the same reactions are seen in the -segments with and without 

 nerve ganglia. Loeb found that if a Planaria was divided in half, 

 the anterior part continued to move quietly as though it had felt 

 no pain. In Gammarus the stomach can be cut away during 

 copulation without interrupting it. Bethe noticed that the 

 abdomen can be cut off a honey-sucking bee without disturbing 

 its occupation. The frog reacts violently to electrical stimulation 

 of the sciatic nerve, but whether it feels much pain is doubtful, as 

 the same reactions take place after decerebration. Herbivora are 

 less sensitive to pain than carnivora. Veterinary surgeons know 

 that horses continue to eat while undergoing an operation, and 

 the rabbit eats directly after serious operations. These and other 

 observations show that the development of sensibility to pain is 

 parallel to the development of the intelligence. In human races 

 sensibility to pain is more developed in proportion as they are 

 more civilised ; in imbeciles, idiots, and dements it is very low. 



Pain, then, is a function of the intelligence, a psychical element 

 superposed upon the subconscious protective, reflexes. A painful 

 sensation produced by a mechanical or thermal agent at the 

 cutaneous periphery may from the teleological point of view be 

 compared with the nauseating taste of a poison. Weber observed . 

 that the temperature which began to produce pain (48 C.) when 

 applied to the skin was the same at which the nerve substance 

 begins to alter. The teleological relation between the painful 

 stimulation of certain afferent paths and certain instinctive 

 reactions witnesses to the protective significance of pain. 



Nevertheless, it cannot be affirmed that pain is an infallible 

 indication of menace to life. There may be severe pain, as in 

 neuralgia, with no manifest lesion of the tissues ; at other times 

 there may be no pain although the tissues are fatally affected, as 

 occurs with an invasion of pathogenic bacteria. This shows that 

 in the world of living beings co-ordination of function to a given 

 end takes place within definite limits, and that the sense-organs, 

 like all the other organs, are adapted to function teleologically 

 during normal relations with the environment, and not in ex- 

 ceptional circumstances. 



Whether the sensations of tickling and itching are to be con- 

 sidered as specific sensations in the same category as sensations 

 of pressure and of pain, or merely as modifications of the latter, 

 is still a matter of controversy. We must first examine the 

 conditions which give rise to them. To arouse tickling in the 

 cutaneous regions provided with hairs, it is only necessary to 

 touch these parts lightly, e.g. by a feather. Even in the parts 

 that have no hair the red of the lips, the nostrils, eyelids, and 



