THE SENSES OF PRESSURE OR TOUCH 741 



fibers.^ These sense-organs possess a high threshold value, i.e., they 

 remain insensitive until a certain upper limit has been reached when 

 their excitation suddenly evokes pain. It is also evident that the latent 

 period elapsing between their stimulation and the sensation, is unusually 

 long; moreover, the quality of this sensation may be materially varied 

 by the simultaneous excitation of other cutaneous receptors. For 

 example, a burning pain results whenever the nerve-endings for pain 

 and heat are activated together and a throbbing pain, whenever the 

 receptors for pain and touch are jointly involved. A peculiar alter- 

 nating character is imparted to the latter sensation by the systolic dis- 

 tention of the blood-vessels, and especially if it meets the resistance of 

 hyperemic and infiltrated tissues. In this group of the composite 

 sensations of pain also belong itching and tickling. Alrutz,^ however, 

 considers these impressions as being evoked by special nerve-endings, 

 which implies that they represent two varieties of one and the same 

 modality of sensation. 



The pain sense forms an important protective mechanism of the 

 body, because it acts as a danger-signal, by means of which abnormal 

 processes may be detected. In many cases, it compels rest and there- 

 by favors the healing of a diseased part. It should also be noted 

 that a certain tissue may become painful by sympathy, i.e., it may 

 develop a decided tenderness in consequence of a diseased condition 

 existing elsewhere. Diseases of the heart or aorta are sometimes 

 associated with pain between the shoulders or with pain radiating out- 

 ward into the arms. Referred sensations of pain are also experienced 

 in inflammatory conditions of the appendix, gall-bladder and biliary 

 ducts. Head^ has shown that the innervation of certain areas of the 

 skin is intimately related to that of certain internal organs, because 

 any particular segment of the spinal cord sends out an autonomic 

 supply of nerve fibers as well as one to the corresponding region of the 

 integument. In certain lesions of the central nervous system, the 

 tactile sensations are sometimes wrongly referred to regions of the 

 body which are actually far removed from those in which they have 

 arisen. Most commonly, however, the corresponding area on the 

 opposite side is believed to be the seat of the excitation. This faulty 

 localization is designated as allochiria and constitutes a frequent 

 tabetic symptom. 



The Temperature Sense. — Contrary to the view that the sensa- 

 tions of heat and cold arise in consequence of the excitation of one and 

 the same end-organ, it is now commonly believed that they represent 

 two distinct modalities which are mediated by separate receptors. 

 Thus, if a pencil-like probe through which warm water is made to cir- 

 culate, is slowly drawn across the surface of the skin, a very decided 

 sensation of heat is generally obtained in one place and none at all 



1 Thunberg, Nagel's Handb. der Physiol., iii, 1905. 



2 Skand. Archiv fiir Physiol., xvii, 1905. 



3 Brain, xvi, 1893. 



