850 



A MANUAL OF PHYSIOLOGY 



Tactile and Common Sensations. 



Under the sense of touch it is usual to include a group 

 of sensations which differ in quality and that in some 

 instances to as great an extent as any of the sensations 

 which are universally considered as separate and distinct 

 but agree in this, that the end-organs by which they are 

 perceived are all situated in the skin, the mucous membrane, 

 or the subcutaneous tissue. Such are the common tactile 

 sensations including pressure and tickling and the sensa- 

 tions of temperature, or, more correctly, of change of 

 temperature. The sensation of pain, 

 although it cannot, perhaps, be abso- 

 lutely separated from these, ought not 

 to be grouped along with them. It is 

 called forth by the stimulation of 

 afferent nerve-fibres in their course ; 

 and it may originate, under certain 

 conditions, in internal organs which 

 are devoid of tactile sensibility, and 

 the functional activity of which in their 

 normal state gives rise to no special 

 sensation at all. The peculiar sensa- 

 tion associated with voluntary mus- 

 cular effort, to which the name of the 

 muscular sense has been given, also 

 deserves a separate place ; for although 

 it may in part depend on tactile sensa- 

 tions set up through the medium of 

 end-organs situated in muscle, tendon, or the structures 

 which enter into the formation of the joints, other elements 

 are, in all probability, involved. 



The simplest form of tactile sensation is that of mere contact, as 

 when the skin is lightly touched with the blunt end of a pencil. 

 This soon deepens into the sensation of pressure if the contact is 

 made closer ; and eventually the sense of pressure merges into a 

 feeling of pain. It is not easy to say whether these various sensa- 

 tions are due to the stimulation of different nervous elements, or to 

 different grades of stimulation of the same elements. But there is 

 some pathological evidence in favour of the former view, e.g., it is 

 said that the sensation of contact is abolished in cicatrices where 



FIG. 327. TACTILE 

 CORPUSCLE FROM 

 SKIN OF FINGER 

 (SMIRNOW). 



(Golgi preparation.) The 

 winding and intersecting 

 black lines are the non- 

 medullated endings of the 

 one or more nerve-fibres 

 that enter the corpuscle. 



