END-BULBS. 



837 



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, fn optical longitudinal section ; B, in transverse section ; n, entering 

 nerve-fibre ; c, nucleated capsule. 



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 nerve- 

 termination 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 papillae 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 termi- 

 nations of the nervi nervorum (Horsley). The spheroidal end-buibs 

 (fig. 395, B, c) are composed of a connective tissue capsule (a) in- 

 closing 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 cor- 

 puscles, spheroidal end-bulbs have not been noticed in any animals below monkeys. 



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 concentri- 

 cally 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 com- 

 plex examples ; the complexity having been produced in the latter by the multiplication of the 

 tunics. In conformity with this view it maybe mentioned that corpuscles resembling the Paci- 

 nian 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 papillae 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 



