548 MANUAL OF PHYSIOLOGY. 



The application of a liquid over 50 C., or below 2 C., causes 

 pain. The suddenness of application to the part, and its dura- 

 tion, and the extent of surface, as well as the previous tempera- 

 ture, have important influence in the amount of pain produced. 



The various kinds of pain which we are all more or less 

 familiar with seem to be related in some way to their mode of 

 production, but we are unable to assign any definite cause for 

 these differences of character. Thus, though such terms as 

 shooting, stabbing, burning, throbbing, boring, racking, dragging 

 pain, have a tolerably clear meaning in general, and may be 

 of diagnostic value, we have only an indistinct knowledge that 

 throbbing depends on excessive vascular distention in a part, 

 that sharp pains are produced by sudden excitation of a sensitive 

 part, and the dull pains by the more permanent stimulation of a 

 part less well supplied with nerves. 



Further, pain, as we think of it, is a complex mental process, 

 made up of many items, such as real sensory impressions, fear, 

 disgust, etc. When a finger is being lanced, patients often cry 

 out most loudly before they are touched with the knife, and 

 show intense feeling when they look at the blood flowing from 

 the wound. 



Hunger and thirst are peculiar and indefinite sensations which 

 are experienced when some time has elapsed since food or drink 

 has been taken. The exact part of the nervous system in which 

 these impressions arise has not been determined. They are, how- 

 ever, said to be associated with peculiar sensations in the stomach 

 and throat respectively. In the same way the venereal appetite, 

 though associated with local sensations, cannot be referred to any 

 one part of the nervous system. 



Nausea is also a sensation which cannot be attributed to any 

 part of the nervous centres. It commonly arises in response to 

 afferent impulses, such as smell, sights, tastes, pharyngeal, gas- 

 tric, or other visceral irritation, and is antagonistic to the appe- 

 tites just named. All the sensations that give rise to or precede 

 nausea are associated in our minds with disagreeable impressions, 

 and no doubt mental operations have much to do with its pro- 

 duction. A child, free from affectation, may be heard to say of 



