TACTILE CORPUSCLES. 



835 



Tactile cells (Merkel). Tactile cells, isolated or in groups, but in the latter case not 

 collected together to form a tactile end-organ, were described by Merkel as occurring in the 

 deeper layers of the epidermis and sometimes in the subjacent true skin over almost the whole 

 of the body (fig. 389 A). In animals they are especially numerous in parts of the skin which are 

 devoid of hairs, as in that which covers the soles of the feet, and on the snout, as well as 

 amongst the epithelium-cells of the hard palate. The cells in question are round or pyriform 

 in shape, and prolonged at one part into the axis-cylinder of a nerve-fibre : in cases where the 

 axis-cylinder is ramified, it may be connected with more than one of these cells. Each cell is 

 stated to be inclosed by a cell-membrane which is continuous with a prolongation of the 

 primitive sheath of the nerve-fibres. When the tactile cells occur in the superficial layers of 

 the cutis vera instead of amongst the cells of the epidermis they are found to be enclosed in a 

 capsule of connective tissue, which is pierced by the axis-cylinder of the nerve-fibre as this 

 passes to apply itself to one of the surfaces of the usually flattened cell. Such a cell, inclosed 

 in a capsule and forming the termination of a nerve-fibre, represents, according to Merkel, the 

 tactile end-organ in its simplest form. The existence of tactile cells such as are described by 

 Merkel is. however, not generally admitted by histologists, but Ranvier has described and 

 figured a mode of termination which is somewhat like that described by Merkel with the 

 exception that the nerve-fibres do not pass directly into the cells, but come into connexion with 

 them through the medium of concavo-convex expansions, to which he has given the name of 

 tactile diftlts or menisci (fig. 389 B). Haycraft states that in the carapace of the tortoise the 

 nerves end in the nuclei of some of the deeper cells of the epidermis. 



Tactile corpuscles or touch-bodies (corpuscula tadus) (figs. 390 to 393). 

 These were discovered by R. Wagner and Meissner in the papillae of the skin of 

 the hand and foot, where they are of an oval shape, nearly ^W f an i nc ^ l n g an( ^ 

 s^o of an inch thick. They may be found in the skin of all parts of the hand and 

 foot, including the bed of the nails, that of the volar surface of the forearm, in the 



Fig. 390. SECTION OF SKIN SHOWING 



TWO PAPILLJE AND DEEPER LAYERS 



OF EPIDERMIS. (Bicsiadecki. ) 



a, vascular papilla with capillary loop 

 passing from subjacent vessel, c ; 6, 

 nerve-papilla with tactile corpuscle, t. 

 The latter exhibits transverse fibrous 

 markings : three nerve-fibres, d, are re- 

 presented as passing up to it : at // 

 these are seen in optical section. 



skin of the nipple in both sexes, 

 in the conjunctiva at the edge of 

 the eyelids, and in the skin of the 

 lips and in the mucous membrane 

 of the tip of the tongue. Similar 

 corpuscles occur in monkeys, but 

 have not been found in animals 

 lower in the scale. One, two, 

 or more medullated nerve-fibres 

 run to the corpuscle and either 

 at once or after winding round 

 it two or more times, pass into 



its interior and become lost to view. The tactile corpuscles were long de- 

 scribed as consisting of a soft structureless core or central part, in which the 

 nerve-fibres were thought to terminate by bulbous enlargements, and of an inclosing 

 capsule of connective tissue, continuous with the perineurium of the nerve, and 

 composed for the most part of transverse or spiral fibres and nuclei, so arranged as 

 to give the little body somewhat the aspect of a miniature fir-cone. It would appear 

 however that a core, like that of the Pacinian corpuscles to be presently described, 

 does not in reality exist in these corpuscles, but that the main substance of the touch- 

 body is composed of connective tissue, prolonged inwards from the capsule in the 



