94 WILLIAM TOWNSENU PORTER. 



been cut through as they rose above or sank below the plane 

 of section. Examination with a high power shows that this 

 is true of only a small number^ and that the myelin at many 

 of the other interruptions is plainly neither cut nor torn. The 

 myelin has here the shape of a cylinder, ending in a truncated 

 cone. When the fibre is seen a little obliquely, the observer 

 looks into the end of this cone and sees that out from it comes 

 a pale, ribbon-like or cylindrical structure, which runs a more 

 or less wavy course until it enters another myelin sheath. It 

 has the look of a constriction elongated by the process of pre- 

 paration. A little search shows that the space which separates 

 two medullary sheaths is often not greater than the breadth of 

 the fibre, and in such cases appears large because the fibres 

 are very broad. In these narrower interruptions the axis- 

 cylinder is usually well shown. In a good section are inter- 

 ruptions not wider than half the breadth of the fibre. On 

 either side is the cone-shaped ending of the medullary sheath ; 

 between the two, looking very small and colourless in contrast 

 with the thick myelin, is the axis-cylinder — straight, of uni- 

 form calibre, and often faintly striped in a longitudinal direc- 

 tion. All the structures lie in parallel planes. The fibre is 

 neither cut nor torn. It is a true constriction, and closely 

 resembles, except that the sheath of Schwann is absent, the 

 Key and Retzius^ drawings of peripheral constrictions in 

 man. 



With a strong immersion system it is possible to see that 

 most of the constrictions are crossed about the middle by a 

 very fine line, which seems to lie closely upon the axis-cylinder, 

 and sometimes appears as an indistinct ring. From the 

 margins of this ring I have sometimes seen a line of equal 

 fineness, passing on either side towards the medullary sheath. 

 These delicate structures are just within the limits of visi- 

 bility. They correspond to the lines found in silvered central 

 fibres, and are probably portions of the ensheathing neu- 

 roglia. No sheath of Schwann is present. Long fusiform 



* ' Studien iu der Anatomic des Nervcusystems uud des Biudegewobes,' 

 Stockholm, 1876, ii, Bd. i, plate vii, figs. 11, 13, 14, 15. 



