NORMAL HISTOLOGY. 



FIG. 100. 



varying complexity : of such specialized structures over a. dozen 

 forms have been described ; since a number of these occur only 

 among the lower vertebrates, the more important types alone will be 

 here considered. 



The special sensory nerve endings may be grouped as 



1. Tactile Cells. 



2. Tactile Corpuscles. 



3. End-Bulbs. 



The tactile cells are found within the deeper layers of the epi- 

 dermis or the adjacent stratum of the corium, and may be either 

 simple or compound; the former are oval 

 nucleated elements, 5-12 p. in size, and resemble 

 ganglion-cells. The centrally-directed portion of 

 the cells is embraced by a peculiar crescentic 

 expansion the tactile meniscus with which 

 the nerve-fibre is probably connected. 



Where two or more such cells are associated to 

 receive the nerve-fibre, a compound tactile cell 

 results ; the corpuscles of Grandry and of Merkel, 

 found respectively in the epidermis of birds and 

 of mammals, are examples of such structures. 

 The medullated nerve-fibre, on meeting the cells, 

 loses its neurilemma and Henle's sheath, these 

 coverings becoming fused with the connective- 

 tissue capsule of the corpuscle ; the axis-cylinder 

 passes between the cells, to become lost within an intercellular 

 flattened tactile disk ; the medullary substance terminates at the 



point where the axis-cylindei 

 FIG. 101. enters the disk. 



The dark stellate figures 

 sometimes seen in gold prep- 

 arations of the epidermis, 

 lying between the epithelial 

 cells, and known as the cells 

 of Langerhans, do not rep- 

 resent nerve-endings, as for- 

 merly claimed, but are prob- 

 ably migrated wandering cells. 

 The more elaborately ar- 

 ranged compound tactile cells 

 and the simpler tactile cor- 

 puscles, such as the spherical 

 end-bulbs of the conjunctiva, 

 are closely related, their differences being but slight; the various 



Termination of sen- 

 sory nerve-fibres within 

 the epidermis ; gold prep- 

 aration : e, deeper layers 

 of epidermis ; c, subja- 

 cent connective tissue ; 

 , nerve-fibrillae pene- 

 trating among the epi- 

 thelial cells. 



Special nerve-endings within the epidermis ; gold 

 preparation: N, nerve-fibre entering the epithelium 

 and dividing into the fibrils which are connected with 

 the tactile disks (m) ; upon these latter rest the tactile 

 cells, c. (After Ranvier.} 



