NORMAL HISTOLOGY. 



FIG. in. Tactile cells of Merkel lying 

 -within interpapillary epithelium ; broken 

 line indicates junction of epithelium 

 and connective tissue layer; () nerve 

 passing into epithelium. X 160. ( Worth- 

 mann.) 



In the skin and the same general plan applies to the mucous membranes 

 the fibres destined for the epidermis lose their myelin coat beneath the 

 epithelium which they enter as vertically coursing nonmedullated fibrils. 

 Within the epithelium they break up into delicate fibrils which undergo 



further division into still finer varicose threads 

 that ramify between the deeper epithelial cells 

 and terminate in free end-knobs. Similar, 

 but far less numerous free-endings, varicose 

 and club-like in form, occur within the con- 

 nective tissue layers of the skin and mucous 

 membranes. Conspicuous ramifications of 

 sensory fibres surround the hair-follicles, 

 lying upon the outer surface of the glassy 

 membrane. 



The tactile cells, found in the deeper 

 layers of the epidermis and sparingly within 

 the subjacent corium, represent a somewhat 

 more differentiated form of intraepithelial 

 terminations and suggest transitions to the 

 more specialized end-organs. In these end- 

 ings the nerve-fibrils terminate in cup-shaped 

 expansions, the menisci, against which rest 

 the tactile (modified epithelial) cells. The 

 latter may be regarded as imperfectly differentiated neuroepithelium, highly 

 differentiated examples of which are seen in the gustatory cells in the taste buds, 

 in the visual cells in the retina and the auditory cells in the organ of Corti. 

 Encapsulated Sensory Endings. In their most highly developed 

 forms, these endings (corpuscula nervorum terminalia) are represented by 

 large special end-organs in which the terminations of the axis-cylinder are 

 enclosed within an elaborate laminated capsule. The latter, however, is 

 more often present as a much simpler and thinner envelope consisting of 

 strands of connective tissue. 



Transition forms between the intraepithelial tactile cells above noted and 

 the more specialized end-organs, always within the connective tissue, are seen 

 in the corpuscles of Grandry (not found in man but conspicuous in the skin 

 covering the bill of many water-fowl), in which the ramifications of the nerve- 

 fibrillae end within a disk-like mass, the tactile 

 disk enclosed between large modified epithelial 

 elements, the tactile cells. 



The group of simpler encapsulated end- 

 organs includes three well-known examples : 

 the tactile corpuscles, the end-bulbs, and the 

 genital corpuscles. 



The Tactile Corpuscles. These 

 bodies, also called the corpuscles of Meissner, 

 are most numerous in the corium of the skin 

 covering the flexor surface of the fingers and 

 toes. They are found also in the integument of other regions possessing 

 sensibility in a high degree, such as the lips, margin of the eyelid, nipple, 

 penis and clitoris, as well as on the dorsum of the hand and foot and the 

 radial surface of the forearm. On the volar surface of the distal segments 

 of the fingers, where they are most numerous, some twenty are found to 

 the square millimeter. The corpuscles occupy the summit of the papillae 



FIG. in. Two corpuscles of Grandry 

 from bill of duck ; nerve is seen enter- 

 ing corpuscle on right. X 265. 



