MODES OF TERMINATION OF NERVE-FIBRES 83 



in some of the papillae of the skin of the hand and foot, in sections of 

 which they may be afterwards studied (see Lesson XXIII.) End- bulbs 

 are found in the conjunctiva of the eye, where in most animals they 

 have a cylindrical or oblong shape (fig. 100), but in man are spheroidal 

 (fig. 101). They have also been found in papillae of the lips and 

 tongue, and in the epineurium of the nerve -trunks, and somewhat 

 similar sensory end- organs also occur in the integument of the external 

 genital organs of both sexes (fig. 99 j. In the skin covering the bill, 

 and in the tongue of certain birds (e.g. duck), a simple form of end- 

 organ occurs, consisting of two or more cells arranged in rows within 

 a capsule, with the axis-cylinder terminating in flattened expansions 

 between the cells (corpuscles of Grandry, fig. 102). 



The Pacinian corpuscles are larger, and have a more complex 

 structure, than the tactile corpuscles and end-bulbs (fig. 103). They 

 are composed of a number of concentric coats arranged like the layers 

 of an onion, and enclosing the prolonged end of a nerve-fibre. A single 

 medullated nerve-fibre goes to each Pacinian corpuscle encircled by 

 a prolongation of perineurium, and within this by endoneurium ; when 

 it reaches the corpuscle, of which it appears to form the stalk, the 

 lamellae of the perineurium expand to form some of the tunics of the 

 corpuscle. The nerve passes on, piercing the other tunics, and still 

 provided with medullary sheath, and surrounded by endoneurium, to 

 reach the centre of the corpuscle. Here the endoneurium is prolonged 

 to form a sort of soft cylindrical core, along the middle of which the 

 nerve-fibre, now deprived of its medullary and primitive sheaths, passes 

 in a straight course as a simple axis-cylinder (fig. 104, c. /.) to termi- 

 nate at the farther end of the core in a bulbous enlargement. Occa- 

 sionally the fibre is branched. 



The tunics of the corpuscle are composed of connective tissue, the 

 fibres of which for the most part run circularly. They are covered on 

 both surfaces with a layer of flattened epithelioid cells, and here and 

 there cleft-like lymph -spaces can be seen between them like those 

 between the layers of the perineurium (see p. 74). 



When sensory nerve-fibres terminate in plexuses, they generally 

 branch once or twice on nearing their termination. The sheaths of the 

 fibres then successively become lost, first the connective tissue or peri- 

 neural sheath, then the medullary sheath, and lastly the primitive 

 sheath, the axis-cylinder being alone continued as a bundle of primitive 

 fibrils (fig. 105, n). This branches and joins with the ramifications 

 of the axis-cylinders of neighbouring nerve-fibres to form a primary 

 plexus. From the primary plexus smaller branches (a) come off, and 

 these form a secondary plexus (e) nearer the surface, generally imme- 

 diately under the epithelium if the ending is in a membrane covered by 

 that tissue. Finally, from the secondary plexus nerve-fibrils proceed 

 and form a terminal plexus or ramification amongst the epithelium - 

 cells (fig. 106, p], the actual ending of the fibrils being generally in 

 little knob-like enlargements (b). Such a mode of ending in terminal 



G2 



