162 HAND-BOOK OF PHYSIOLOGY. 



lungs, exciting, through the common sensory nerves, merely modifications 

 of common feeling; and at the same time they excite the sensations of 

 smell and of taste. 



SPECIAL SENSES TOUCH. 



Seat. The sense of touch is not confined to particular parts of the 

 T^ody of small extent, like the other senses; on the contrary, all parts capa- 

 ble of perceiving the presence of a stimulus by ordinary sensation are, 

 in certain degrees, the seat of this sense; for touch is simply a modifica- 

 tion or exaltation of common sensation or sensibility. The nerves on 

 which the sense of touch depends are, therefore, the same as those which 

 confer ordinary sensation on the different parts of the body, viz., those 

 derived from the posterior roots of the nerves of the spinal cord, and the 

 sensory cerebral nerves. 



But, although all parts of the body supplied with sensory nerves are 

 thus, in some degree, organs of touch, yet the sense is exercised in per- 

 fection only in those parts the sensibility of which is extremely delicate, 

 e.g., the skin, the tongue, and the lips, which are provided with abun- 

 dant papillae. A peculiar and, of its own kind in each case, a very acute 

 sense of touch is exercised through the medium of the nails and teeth. 

 To a less extent the hair may be reckoned an organ of touch; as in the 

 case of the eyelashes. The sense of touch renders us conscious of the 

 presence of a stimulus, from the slightest to the most intense degree of its 

 action, by that indescribable something which we call feeling, or common 

 sensation. The modifications of this sense often depend on the extent 

 of the parts affected. The sensation of pricking, for example, informs 

 us that the sensitive particles are intensely affected in a small extent; 

 the sensation of pressure indicates a slighter affection of the parts in the 

 greater extent, and to a greater depth. It is by the depth to which the 

 parts are affected that the feeling of pressure is distinguished from that 

 of mere contact. Schiff and Brown -Sequard are of opinion that common 

 sensibility and tactile sensibility manifest themselves to the individual by 

 the aid of different sets of fibres. Sieveking has arrived at the same con- 

 clusion from pathological observation. 



Varieties. (a) The sense of touch, strictly so-called (tactile sensi- 

 bility), (b) the sense of pressure, (c) the sense of temperature. These 

 when carried beyond a certain degree are merged in (d) the sensation of 

 pain. 



^ Various peculiar sensations, such as tickling, must be classed with 

 pain under the head of common sensations, since they give us no infor- 

 mation as to external objects. Such sensations, whether pleasurable or 

 painful, are in all cases referred by the mind to the part affected, and 

 not to the cause which stimulates the sensory nerves of the part. The 



