378 



THE SKIN AND THE SENSE-ORGANS. 



a. [The nerve-plexus of the epithelium is very incomplete as 

 compared with the plexus found in the epithelium of the cornea ; 

 here and there a few non-medullated fibres have been traced a short 

 distance between the epithelial cells. No such mode of nerve- 

 terminations in the epithelium as is described by Eberth and Macul- 

 lum in the tadpole can be found in the adult.] 



b. The touch-spots. What knowledge we possess of these 

 organs is due to the labours of Leydig, Ciaccio, Eberth, and more 

 particularly those of Merkel. 



At the base of each papilla, which contains such a touch-organ, 

 is found a number of colourless, flattened cells, arranged either in a 

 single layer, or heaped up and connected with very fine nerve-fibres. 



Fig. 236. 



II. 



I. Lateral sense-organ of tadpole of frog. Half-schematic. 



c Central zone (nerve-epithelium). 



Ml Memhrana limitans. 



p Peripheral zone (sustentacular cells). 



Jt Hjab'ne tube. 



II. Touch-corpuscle from the sheath of a digital tendon of the frog ; 

 after Lowe. Schieck, Oc. O., Obj. 9. 



The flat surface of the cells is parallel to the surface of the body, 

 and they do not form a separate or circumscribed body, they are 

 therefore better spoken of as touch-spots than as true touch-bodies 

 (Merkel). The larger organs are found in the papillae, but smaller 

 ones may be found on any part of the skin. 



These organs are best seen in the prominence or swelling upon the 

 supplemental toe during the breeding season ; they are numerous 

 on the dorsal surface of the trunk, but occur most freqiiently on 

 the under surface of the hinder feet (Planta pedis). After the 



