i CUTANEOUS SKXSIBILITV 17 



average, therefore, ll'-l.". cold spots anil 1-2 heal spots per 

 sij. em. 



1'dix round that in the hair-clad regions oi' the skin, which 

 he estimates at about 95 per cent of the whole, the pressure 

 points coincide with the hair papillae; other pressure points that 

 ran he detected here and there where there are no hairs probably 

 correspond to rudimentary hair papillae. Hut the tactile surfaces 

 proper, where the touch spots are closely arranged, are found in 

 the regions that have no hairs -particularly the tips of the 

 lingers, palm of the hand and sole of the foot, red part of the lips, 

 tip of the tongue, etc. The number of pressure points, according 

 to v. Frey, averages 25 to each sq. cm., except on the head. 



The number of pain points has not yet been estimated. On 

 the back of the hand v. Frey found 100-200 in every sq. cm. 



Once the position of the sensory spots on any part of the skin 

 is fixed by means of fast colours, it is easy not only to identify 

 them at any time, but also to verify on them Miiller's law of 

 the specific energies by showing that they react by the same form 

 of sensation (warmth, cold, pressure or pain) when excited not only 

 by adequate but also by inadequate stimuli. Sensations of cold, 

 e.g., are obtained by exciting the corresponding spots not only with 

 a cold point but also with a mechanical or electrical stimulus, or 

 with a point heated to 45 v. Frey's j)ttr<i</<>.i-ie(tl sensation of 

 cold. 



The legitimate conclusion from these results is that the skin is 

 provided with at least four distinct sets of sensory nerves, for the 

 sensations of cold, warmth, contact or pressure, and pain ; that 

 these nerves terminate within the skin in special peripheral sense- 

 organs ; and, lastly, that the sensitive points of the cutaneous 

 surface correspond to these sense-organs in the layers below them. 



V. Before attempting to solve the question whether four 

 different organs or terminal corpuscles correspond with the four 

 forms of cutaneous sensation, we must refer to the latest morpho- 

 logical work on the nerve-endings in the skin. 



The sensory nerve-til ires that innervate the skin form a deep 

 nerve-plexus in the subcutaneous panniculus adiposus. Most of 

 the fibres of this plexus run towards the surface of the skin, 

 and after crossing the reticular layer of the cutis reach the 

 sul (papillary layer, where they form a second plexus less rich in 

 fibres, the so-called superfieial cutaneous nerve-plexus. A deep 

 vascular network corresponds to the deep nerve-plexus; a super- 

 ficial vascular network to the surface plexus. 



Fibres are given off by the deep plexus which terminate after 

 a short course in special corpuscles or peripheral sense-organs 

 situated in the panniculus adiposus. From the superficial nerve- 

 plexus still more numerous fibres branch off to end in special 

 corpuscles in the different layers of the cutis --the reticular, 



VOL. IV C 



