SENSE ORGANS OF THE INTEGUMENT 



257 



they are simplest on the glans penis and glans clitoridis. It is 

 doubtful whether they occur on hairy portions of the skin, though 

 it is certain that the hairs (and more especially the vibrissre, cf. p. 30), 

 have a sensory function, which is very marked in the case of those 

 on the wings and ears of Bats. 



Circumscribed and well-innervated areas, each composed of a 

 cap of thickened and curiously modified epithelium covering a 

 dermal papilla, have been observed in numerous Mammals close to 

 the hairs : these hair-discs represent a special kind of sensory 

 apparatus. In Man, each disc is a small, rounded structure situ- 

 ated in the acute angle between the oblique hair-shaft and the 

 skin-surface : in a corresponding position in the obtuse angle 

 opposite to it is another well-marked, smooth area, probably corre- 

 sponding to a scale-rudiment. These two structures, together with 

 the other accessory organs of the hair (glands, muscles, nerves, 

 vessels, &c.), constitute a well-defined "hair-urea" which is probably 

 the morphological equivalent of the reptilian scale. 



d. Club-shaped or lamellar corpuscles. 

 (Pacinian corpuscles.) 



In Lizards and Snakes club-shaped corpuscles are present in 

 addition to the above-described tactile organs, occurring chiefly in 

 the region of the lips and teeth and also on the body (e.g. Lacerta) ; 

 they have an elongated, oval form, and their structure is simple. 



A B 



FIG. 188. PACINIAN CORPUSCLE FROM MESORECTUM OF KITTEN A, two 

 days, and B, three days old. (After Guido Sala.) 



In A the nerve-network is seen surrounding the main fibre. In B, knob-like 

 outgrowths are shown on the fibre at its distal end. 



When more highly developed, the interior of each corpuscle 

 shows the continuation of the axis-fibre of the nerve surrounded 



