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 disks 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 tactûs)-(figs. 390 to 393). These were discovered by R. Wagner and Meissner in the papillæ of the skin of the hand and foot, where they are of an oval shape, nearly

of an inch long and

6 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

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

form of imperfect membranous septa (figs. 392, 393 A), between which are supported the convolutions and ramifications of the nerves, and the enlargements in which the branches of the axis-cylinder eventually end (small tactile cells according to Merkel). These terminal enlargements, which are either pyriform or globular in shape, are always placed near the capsule, and in small tactile corpuscles may occasionally project beyond it. On entering the corpuscle the nerve-fibres for the most part lose

Fig. 392.-ANOTHER CORPUSCLE, TREATED WITH OSMIC ACID, SEEN IN LONGITUDINAL SECTION. (Flemming, from a preparation by Fischer.)

1, entering nerve-fibre, medullated; 2, 2, the same cut variously within the corpuscle; 3, 3, clear spaces around the fibre (perhaps homologous with the core of the cylindrical end bulbs); 4, 4, nuclei of the transverse and spirally-disposed cells of the corpuscle.

Fig. 393. TACTILE CORPUSCLES FROM THE PALM OF THE HAND, SEEN IN SECTION (Merkel).

A, Longitudinal section showing the interior traversed by connective tissue septa derived from the capsule; the nerve-fibres are cut across. B, transverse section at the point of entrance of a nerve-fibre, showing the axis-cylinder branching. Other nerve-fibres are cut obliquely.

their medullary sheath, but some retain it for a short while, or it may reappear here and there in the course of the fibres. The axis-cylinders, which are often varicose, have, as before intimated, a convoluted course before ending in their terminal enlargements (fig. 391).

The absence of a central core such as is found in the Pacinian corpuscles, and in some end-bulbs, was first pointed out by Langerhans and afterwards by Thin. The latter observer stated as the result of his observations, that tactile corpuscles could be divided into simple or compound, according as they receive one or a greater number of nerve-fibres; each nerve-fibre passing to one distinct corpuscle, and the larger corpuscles being compounded of two or more simple ones. On the other hand, even in the same papilla, several small corpuscles may occur near to but distinct from one another.

End-bulbs. If the conjunctiva of the calf or of certain other animals is carefully spread out and examined under the microscope, many of the medullated nerves which course in different directions in the membrane may be seen to end in very small oval or elongated corpuscles, into the interior of which the axis-cylinder of the nerve-fibre passes, surrounded by a soft homogeneous core, to end near the extremity of the corpuscle, with a rounded or dilated termination. The core with its contained fibre is inclosed in a simple nucleated capsule composed of flattened cells. The medullary sheath ceases abruptly at the entrance of the nerve, whereas the primitive sheath appears to be continued over the core, and to line the capsule. These

Fig. 394.-CYLINDRICAL END-BULBS FROM THE CONJUNCTIVA OF THE CALF.

(Merkel.)

A, in optical longitudinal section; B, in transverse section; n, entering nerve-fibre; c, nucleated capsule.

n

so-called cylindrical end-bulbs were discovered by W. Krause, and they have been found not only in the conjunctiva of different mammals, but also forming the most common mode of nervetermination in various parts of the skin and here and there in the mucous membrane of the mouth. Terminal corpuscles of this exact nature and form, have however hitherto not been observed in the conjunctiva of man nor of apes, but their place is here supplied by the small spheroidal end-bulbs of Krause (figs. 395, 396), which have also been found in man in the papillæ of the skin covering the lips, in the mucous membrane of the cheeks, soft palate, tongue, epiglottis, nasal cavities, lower end of the rectum, and in that of the glans penis and clitoridis. Corpuscles which are closely allied to, if not identical with these, are also found in the epineurium of nerve trunks, where they constitute the terminations of the nervi nervorum (Horsley). The spheroidal end-bulbs (fig. 395, B, C) are composed of a connective tissue capsule (a) inclosing a core formed of numbers of polygonal and elongated cells, which give the core a granular aspect; amongst the cells of the core the axis-cylinder terminates. Sometimes the small medullated fibre which passes to each spheroidal end-bulb, divides into two or more branches before reaching the bulb, and the branches may be twisted around one another on their passage towards the organ (B). The capsule is continuous with the sheath of Henle of the nerve-fibre, and internally it is closely invested with a nucleated membrane, prolonged from the primitive sheath. Like the tactile corpuscles, spheroidal end-bulbs have not been noticed in any animals below monkeys.

B

The cylindrical end-bulbs closely resemble the central part or core of a Pacinian body divested of all but its innermost tunic, and, to complete the resemblance, flattened concentrically arranged cells are described by Merkel as forming the core of the end-bulb as well as that of the Pacinian. In short, it would seem as if the little bodies in question represent the simplest of the type of terminal corpuscles of which the Pacinian corpuscles are the most complex examples; the complexity having been produced in the latter by the multiplication of the tunics. In conformity with this view it may be mentioned that corpuscles resembling the Pacinian bodies are frequently found, especially in the lower animals, in which the tunics are few in number and the corpuscles correspondingly smaller. On the other hand, the round end-bulbs approach more nearly to some of the tactile corpuscles in structure, those, namely, of a simple kind, such as are met with in many parts of the integument, and those form a transition to the more complicated tactile corpuscles which occur in the papillæ of the human hand. At the same time it cannot be supposed that there is any fundamental difference in the two kinds of end-bulb, although the arrangement of the cells in the core and the course taken by the

nerve-fiore is somewhat different, since we see that in different animals those of the one kird are replaced by those of the other kind.

Large end-bulbs of a rounded oval form have been found in the synovial membrane of certain joints in man (e.g., those of the fingers), and also in the

A, Ramification of nerve fibres in the mucous membrane, and their termination in end-bulbs, as seen with a lens; B, an end-bulb more highly magnified; a, nucleated capsule; b, core, the outlines of its component cells are not seen; c, entering fibre branching and its two divisions passing to terminate in the core at d; C, an end-bulb treated with osmic acid, showing the cells of the core better than B; a, the entering nerve-fibre; b, capsule with nuclei; c, c, portions of the nerve-fibre within the end-bulb, the ending of the fibre is not seen; d, e, cells of the core.

articular synovial membranes of several mammals. They are somewhat flattened, have a large granular core beset with nuclei, and receive from one to four medullated nerve-fibres, which terminate within them in fine convoluted and ramified nonmedullated filaments (fig. 397). They are distinguished by the name of articular nerve-corpuscles or articular end-bulbs.

What appear to be a modification of end-bulbs were discovered by W. Krause in certain parts of the external generative organs, both in the male and female (especially the glans penis and clitoridis), and were named by him genital corpuscles. These corpuscles are constructed generally like the end-bulbs, but are characteristically constricted or subdivided by connective tissue septa into from two to six knob-like portions, which gives the whole corpuscle a mulberry-like aspect. From one to four medullated fibres enter the genital corpuscle, and their axis-cylinders usually break up within it into a large number of fine pale terminal fibres. Their size varies greatly, some of them being no larger than ordinary end-bulbs, others several times as large. In the simplest of these structures the axis-cylinder of the nerve-fibre entering at one pole of the somewhat oval corpuscle (fig. 398) may either pass straight or with

one or two bendings through the corpuscle, and end by a tapering (A) or by a dilated extremity (B) near the opposite pole (often projecting beyond the general body of the organ, as in B); or it may be much convoluted and ramified in its passage,

Fig. 396.-END-BULB OF THE HUMAN CONJUNCTIVA, TREATED WITH 3 P.0. ACETIC ACID AND 1 P.C. OSMIC ACID. 50. (W. Krause.)

Fig. 397.-ARTICULAR CORPUSCLE FROM PHALANGEAL JOINT IN MAN. ACETIC ACID PREPARATION. (W. Krause.) spo.

n, two medullated nerve-fibres entering the corpuscle.

so as to render it a matter of difficulty to trace its course and mode of termination (fig. 398 C). The arrangement of the cells in these corpuscles seems to vary considerably. Sometimes they are chiefly collected at the exterior, leaving the part

Fig. 398.-A and B, GENITAL CORPUSCLES FROM THE CLITORIS OF THE RABBIT (Izquierdo); C, FROM THE HUMAN CLITORIS (W. Krause).

traversed by the axis-cylinder free from cells and of an obscurely fibrous appearance, concentrically striated in transverse section (G. Retzius); but in others there is an agglomeration of cells in the centre, as in the spheroidal end-bulbs of the human conjunctiva; this is, however, denied by Retzius.