SENSORY AND TACTILE SENSATIONS. 923 



that receive tactile impressions, and are consequently designated 

 nerves of touch or tactile fibers. Tactile sensations include the per- 

 ceptions of temperature and of pressure. Some observers assume that 

 the sensory and tactile nerves possess separate end-organs and nerve- 

 fibers, and that they likewise have special perception-centers in the brain, 

 although little of a definite nature is known in this connection. This 

 view is supported : ( i ) By the fact that both sensory and tactile sensations 

 are not excited at the same time in all of the areas endowed with feeling. 

 Tactile sensations (including pressure and temperature) are transmitted 

 only by the coverings of the external integument, the oral cavity, the 

 entrance and floor of the nasal cavity, the pharynx, the end of the 

 rectum, and the urogenital openings; feeble, indistinct sensations of 

 temperature are appreciated also in the esophagus. On the other 

 hand, tactile sensations are wanting in all the viscera, as experiments on 

 men with fistulas of the stomach, intestine, and bladder teach; in these 

 situations pain alone can be excited. (2) The paths for the tactile and 

 sensory nerves are far apart in the spinal cord ; this renders probable the 

 assumption that also their central and peripheral extremities are dis- 

 tinct. (3) The reflexes (tactile and painful) excited by the two kinds 

 of nerves are probably controlled or inhibited respectively by special 

 central organs. (4) Under pathological conditions and under the in- 

 fluence of narcot- 

 ics, the one kind of 

 sensation may be 

 abolished , while 

 the other is pre- 

 served. 



Bier and Hilde- 



brandt found in them- ^^^^^^^^ ^^^^ Branches passing to the 



selves that after injec- "^S^J^ epithelium. 



t l n / f i C Cain int ? ^ Nerve-trunks, 



the aural cavity of 



the spinal COrd, the FIG. 338. Nerve-endings in the Corneal Epithelium, 



sensation of pain was 

 abolished, while the 



sensation of touch persisted; sensations of cold and heat were preserved, but 

 intense heat caused no pain. According to another view, the sensation of pain 

 belongs to the nerves both of pressure-sense and of common sensation, and rep- 

 resents only an increase in the irritation of these nerves. 



The nerves of sensation must be subjected to relatively strong irrita- 

 tion in order that pain may be excited. The irritant may be mechanical, 

 electrical, thermal, chemical, or somatic, the last in connection with 

 inflammatory processes, nutritive disorders, etc. The nerves are sensitive 

 to irritations, not only at the peripheral extremity, but also through- 

 out their entire course ; and the central extremity is sensitive to irrita- 

 tion by pain The pain, however, is, according to the law of peripheral 

 perception, always referred to the periphery. 



The tactile nerves can convey pressure-sensations only as a re- 

 sult of moderately strong, mechanical irritations causing differences in 

 pressure, and temperature-sensations as a result of thermal stimuli; and 

 in both instances only when the peripheral end-organs are irritated. If 

 pressure or cold be applied in the course of a nerve -trunk, for example to 

 the ulnar in the depression in the inner condyle, sensations of pain- 

 never of touch are excited in the peripheral distribution. All intense 



