IO8 NORMAL HISTOLOGY. 



find, on examining with high powers, irregularly 

 scattered along each interannular segment, delicate 

 oblique lines or fissures, called the incisures of 

 Schmidt, which seem to pass from the neurilemma 

 at the surface inward to the axis cylinder, obliquely 

 through the medullary sheath. Their significance 

 is not, as yet, definitely determined. Medullated 

 nerve-fibres vary greatly in diameter. 



Connective Tissue of the Nerves. The nerve-fibres 

 are bound together by connective tissue, to form 

 larger and smaller nerve-fascicles, which, singly or in 

 bundles, we usually call simply nerves. If we follow 

 the nerves outward toward their peripheral termina- 

 tions, we find that they divide and subdivide, be- 

 coming, as they do so, smaller and smaller, until we 

 finally come to nerves which consist of a single 

 fibre. These single nerve-fibres do not lie free in 

 the tissues, but are enclosed in a distinct sheath, 

 called Plenle's sheath, which is a tube formed of a 

 single layer of endothelial cells, placed edge to edge, 

 and cemented together. Between the sheath and 

 the nerve-fibre is a narrow space, which, under nor- 

 mal conditions, is filled with lymph. 



Having become acquainted with this simple struc- 

 ture of the single terminal nerves, let us follow them 

 backward. We find, as we do so, that as they be- 

 come larger by the junction of several fibres a small 

 amount of fibrillar connective tissue appears be- 

 tween the fibres within Henle's sheath, and that the 



