370 SPECIAL PHYSIOLOGY. 



In parts of the body, in which there is incomplete paralysis of sen- 

 sation, the sense of temperature remains, or is the last to disappear ; 

 so that paralyzed parts, which are no longer sensible to pressure or 

 pricking, still remain sensible to the influence of heat and cold. This 

 may be explained, either by supposing the existence of special sets 

 of nerves for the conveyance of thermal impressions, or the occur- 

 rence of peculiar changes in the path of the proper tactile sensations 

 in the cord, or in the corresponding sensorial centres in the cerebrum. 



The mucous membrane of the alimentary canal generally, is inca- 

 pable of receiving impressions of thermal differences, though these are 

 felt in the mouth, pharynx, and, for a short distance, down the oeso- 

 phagus. In the stomach or intestines, cold or hot water produces cor- 

 responding sensations of cold or heat, only when the temperature of 

 the adjacent skin of the abdomen is itself lowered, or elevated, by the 

 conduction of heat to or from it, from or to the alimentary canal in 

 which the hot or cold water is contained. 



Subjective sensations of heat or cold are very common in cases of 

 disease. That of cold, in the stage of ague, and that of heat, in 

 febrile conditions of the system, bear no relation to the actual temper- 

 ature of the body. The former is supposed to be due to a contracted 

 state of the muscular coats of the bloodvessels, and of the non-striated 

 muscular fibres of the skin, which conditions diminish the supply of 

 blood to the part. The heat in fever, is attributed to the increased 

 activity of the circulation, and of the metamorphosis of the tissues. 



Sensations of heat may be confounded with tactile impressions, 

 even in those parts of the body in which sensibility is most highly 

 developed. (Fick and Wunderli.) 



It is supposed that the reason why the only sensation experienced 

 on immersing the hand or foot in mercury, or a warm fluid, as that of 

 -a ring around the limb at the surface of the liquid, is due to the fact 

 that the portion of the limb immersed, being subjected to uniform pres- 

 sure, its papillae are not excited, but only those corresponding to the 

 line at which the different pressures exerted by the air and by the 

 fluid, meet. 



The Organs and Sense of Touch in Animals. 



Amongst the Vertebrata in Mammalia, as in Man, the whole surface of the 

 T)ody possesses not only common sensibility and a general sense of touch, ex- 

 cept in those species in which, as in the Armadillo, the integument presents a 

 thick, horny, or bony covering, but the proper tactile sense is principally exer- 

 cised by parts provided with nervous papillae. In most Quadrumana. the 

 tips of the toes and. fingers, where the sense of touch is most acute, are abun- 

 dantly supplied with papillae ; and the under surface of the prehensile tail of 

 certain monkeys, which likewise has many papillae, is also a tactile organ. In 

 many Rodentia .the pulps of the digits are highly sensitive. In the Bat the 

 sense of touch is extraordinarily developed in the wings; by which means it 

 can avoid objects daring its night, even when the eyes are extirpated. (Spal- 

 lanzani.) The whiskers or vibrissoe of the Carnivorous tribes, especially in the 

 eat and seal, -and also those of many rodents, as in the rabbit and hare, are 

 endowed with very acute tactile sensibility. The bulbs of these vibrissffl are 

 very large, and each receives a nerve, often of considerable size, derived from 

 offsets of the infra-orbital branch of the fifth cranial nerves ; when these 

 whiskers are cut off, the sense of touch in the animal is seriously impaired. 



