ioi6 



HUMAN ANATOMY. 



Fig. 867. 



Tactile cells of Merkel lying within inter- 

 papillary epithelium ; broken line (c) indicates 

 junction of epithelium and connective tissue 

 layer; (w) nerve passing into epithelium. X 160. 

 {IVorthmann.) 



Fig. 868. 



The tactile cells of Merkel, found in the deeper layers of the epidermis, 

 represent a somewhat more differentiated form of intraepithelial terminations and 



suggest transitions to the more specialized end- 

 organs. In these endings the nerve-fibrils 

 terminate in cup-shaped expansions or menisci, 

 against which rest the modified epithelial cells. 

 The latter may be regarded as an imperfectly 

 differentiated neuroepitheliiivi, examples of 

 which are seen in the gustatory cells in the 

 taste buds and in the highly specialized visual 

 and auditory cells in the retina and in the 

 organ of Corti respectively. 



Encapsulated Sensory Endings. — In 

 their most highly developed forms these end- 

 ings (corpuscula nervorum terminalia) are 

 represented by relatively 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 fibrous 

 tissue. 



Transition forms between the intraepithelial tactile cells above noted and the 

 more specialized encapsulated 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 and in 

 the tongue of many water-fowl), in which the nerve 

 ends in a disc-like expansion enclosed between large 

 modified epithelial cells and the neuromuscular and 

 neurotendinous end-organs, presently to be described 

 (page 1020). 



The group of simpler encapsulated endings 

 includes three well-known examples : the end-bulbs 

 and the genital corpuscles of Krause and the cor- 

 puscles of Meissner, all of which possess a common 

 structural plan — interwoven telodendria embedded 

 within a semifluid interfibrillar substance and surrounded by a thin fibrous envelope. 



The End-Bulbs of Krause. — These endings 

 include a variety of irregularly spherical or ellipsoidal 

 bodies found in the edge of the eyelid, the conjunctiva 

 and corneal margin, the lips and the oral mucous 

 membrane, the glans penis and clitoridis and probably 

 other parts of the integument highly endowed with 

 sensibility. Within the conjunctiva, as described by 

 DogieP, they lie superficially placed within the con- 

 nective tissue near the summit of the papillae and 

 folds, when such elevations exist, but always close 

 beneath the epithelium. They vary considerably in 

 size, often being small (.002-. 004 mm.), but some- 

 times measuring from .05-. 10 mm. in diameter. 

 Usually a single nerve-fibre, exceptionally two or even 

 more, enters each bulb, losing its medullary sheath as 

 it pierces the thin fibrous capsule. Within the latter 

 the nerve, now represented by the naked axis-cylinder, 

 divides into from two to four branches, which, after 

 describing several annular or spiral turns, give off 

 varicose fibrils that undergo further division, the terminal threads forming a more or 

 less intricate maze within the semifluid substance enclosed by the fibrous capsule. 



' Archiv f. mik. Anat., Bd. xliv., 1895. 



Two corpuscles of Grandry from 

 bill of duck ; nerve is seen entering 

 corpuscle on right. X 265. 



Fig. 869. 



Two end-bulbs of Krause from human 

 conjunctiva. {Dogiel.) 



