THE SENSES OF PRESSURE OR TOUCH 



735 



fiber which may be single or split up into a delicate ramification. These end-organs 

 are very numerous in the subcutaneous connective tissue. The corpuscle of Herbst 

 which is chiefly found in birds, is somewhat smaller, but very similar in structure 

 to the preceding. Another cutaneous receptor has been described by Krause (1860). 

 It is globular in shape and its central region is taken up by the arborization of the 

 nerve fiber. Equally characteristic and suggestive of their function are the corpus- 

 cles of Grandry and Merkel. They consist of two or several hemispherical cells with 

 flattened surfaces, between which the nerve fiber is expanded. Their height 

 measures 15/J. and their breadth 50/j.. The composite type of these corpuscles is 

 present in great numbers in the skin of the bill and tongue of birds. Several 

 different types of endings have been found around and in the immediate vicinity 

 of the roots of hairs. 



FIG. 362. PACCINIAN CORPUSCLES FROM THE PERITONEUM OF A CAT. 

 Bohm-Davidoff-Huber's Histology.) 



(After Sola, from 



Methods Used to Evoke Tactile Sensations. The skin is exposed 

 to influences which are capable of eliciting several kinds of sensations, 

 namely, pressure, touch, pain, cold, warmth, tickling, and others of a 

 more composite type. The sensations of pressure and touch are depen- 

 dent upon mechanical stimuli and find their origin in a displacement 

 of the surface layers of the skin. 1 They represent sensations caused by 

 different grades of the same mechanical impact, but the displacement 

 need not take place in an inward direction but may also result in 

 consequence of pull upon the surface. In the former case we obtain 

 a positive and in the latter a negative imprint. 



The tactile sensations are usually tested by means of an instrument, 

 which is known as an esthesiometer. In its simplest form it con- 

 sists of a hair or fiber of glass-wool attached to a handle, the tip of which 



1 Frey and Kiesow, Zeitschr. fur Psych, und Physiol. der Sinnesorgane, xx, 

 1899. 



