838 COMMON SENSATION PAIN. 



different temperatures act simultaneously and side by side, the impressions are apt 

 to become fused, especially when the two areas are very near each other. 



6. Practice improves the temperature sense ; congestion of venous blood in the 

 skin diminishes it ; diminution of the amount of blood in the skin improves it (M. 

 AUberg). When large areas of the skin are touched, the perception of differences 

 is more acute than with small areas. Rapid variations of the temperature produce 

 more intense sensations than gradual changes of temperature. Fatigue occurs soon. 



Illusions are very common : 1. The sensations of heat and cold sometimes alternate in a 

 paradoxical manner. When the skin is dipped first into water at 10 C. we feel cold, and if it 

 be then dipped at once into water at 16 C, we have at first a feeling of warmth, but soon again 

 of cold. 2. The same temperature applied to a large surface of the skin is estimated to be 

 greater than when it is applied to a small area, e.g., the whole hand when placed in water at 

 29 5 C. feels warmer than when a finger is dipped into water at 32 C. 3. Cold weights are 

 judged to be heavier than warm ones. 



Pathological. Tactile sensibility is only seldom increased (hyperpselaphesia), but great 

 sensibility to differences of temperature is manifested by areas of the skin whose epidermis is 

 partly removed or altered by vesicants or herpes zoster, and the same occurs in some cases of 

 locomotor ataxia ; while the sense of locality is rendered more acute in the two former cases and 

 in erysipelas. An abnormal condition of the sense of locality was described by Brown-Sequard, 

 wh*re three points were felt when only two were applied, and two when one was applied to the 

 skin. Landois finds that in himself pricking the skin of the sternum over the angle of 

 Ludovicus is always accompanied by a sensation in the knee. [Some persons, when cold water 

 is applied to the scalp, have a sensation referable to the skin of the loins (Stirling).] A 

 remarkable variation of the sense of locality occurs in moderate poisoning with morphia, 

 where the person feels himself abnormally large or greatly diminished. In degeneration of the 

 posterior columns of the cord, Obersteiner observed that the patient was unable to say whether 

 his right or left side was touched (" allochiria "). Ferrier observed a case where a stimulus 

 applied to the right side was referred to the left, and vice versd. 



Diminution and paralysis of the tactile sense (Hypopselaphesia and Apselaphesia) occur 

 ither in conjunction with simultaneous injury to the sensory nerves, or alone. It is rare to 

 find that one of the qualities of the tactile sense is lost, e.g., either the tactile sense or the 

 sense of temperature a condition which has been called "partial tactile paralysis." Limbs 

 which are "sleeping" feel heat and not cold (Herzen). 



429. COMMON SENSATION PAIN. By the term common sensation we 



understand pleasant or unpleasant sensations in those parts of our bodies which are 

 endowed with sensibility, and which are not referable to external objects, and 

 whose characters are difficult to describe, and cannot be compared with other 

 sensations. Each sensation is, as it were, a peculiar one. To this belong pain, 

 hunger, thirst, malaise, fatigue^ horror, vertigo, tickling, well-being, illness, the 

 respiratory feeling of free or impeded breathing. 



Pain may occur wherever sensory nerves are distributed, and it is invariably 

 caused by a stronger stimulus than normal being applied to sensory nerves. Every 

 kind of stimulation, mechanical, thermal, chemical, electrical, as well as somatic 

 (inflammation or disturbances of nutrition), may excite pain. The last appears to 

 be especially active, as many tissues become extremely painful during inflammation 

 {e.g., muscles and bones), while they are comparatively insensible to cutting. Pain 

 may be produced by stimulating a sensory nerve in any part of its course, from its 

 centre to the periphery, but the sensation is invariably referred to the peripheral 

 end of the nerve. This is the law of the peripheral reference of sensations. 

 Hence, stimulation of a nerve, as in the scar of an amputated limb, may give rise 

 to a sensation of pain which is referred to the parts already removed. Too violent 

 stimulation of a sensory nerve in its course may render it incapable of conducting 

 impressions, so that peripheral impressions are no longer perceived. If a sufficient 

 stimulus to produce pain be then applied to the central part of the nerve, such an 

 impression is still referred to the peripheral end of the nerve. Thus we explain the 

 paradoxical anaesthesia dolorosa. In connection with painful impressions, the 

 patient is often unable to localise them exactly. This is most easily done when a 

 small injury (prick of a needle) is made on a peripheral part. When, however, the 



