STRUCTURE AND ORGANISATION OF NERVE 



41 



S- 



If these excessively fine elements of the olfactory nerve, which 

 resemble the "fibrils" of Kemak's fibres in structure and appear- 

 ance, are thus independent and non-medullated fibres, they represent 

 in some degree the lowest and least developed form of nervous tissue. 

 Kemak's fibres are higher in the scale, since they present bundles 

 of fibrils, which possess (even if incompletely) a sheath of Schwann : 

 while the most developed non-medullated fibres again appear 

 as an axis - cylinder completely surrounded with 

 a sheath of Schwann, as, typically, in the peri- 

 pheral nerves of the lower, and even, at a certain 

 developmental stage, of the higher vertebrates 

 (Petromyzon, Ampliioxus, Cyclostom,ci). The axis- 

 cylinder, to the finer structure of which we shall 

 return later, is here immediately surrounded on 

 all sides by the sheath of Schwann. This is tubular 

 and transparent, and betrays its cellular formation 

 only in the presence of long nuclei upon the inner 

 surface ; the upper surface is often invested with a 

 delicate membrane of fibrillated connective tissue 

 (" Henle's sheath "), which may be regarded as part 

 of the connective tissue (perineurium) that binds a 

 number of fibres into a nerve-trunk (Fig. 152). 



All nerves consisting of these kinds of elements 

 differ essentially, even to the unaided eye, from 

 those composed exclusively, or in great part, of 

 medullated fibres, the complicated structure of 

 which will be described below. Non-medullated 

 nerves, in consequence of the transparency of the 

 single fibres and their investment, are always clear, 

 and grayish in colour, and often, especially in 

 invertebrates, of an almost gelatinous consistency. FIG. 

 Medullated nerves, on the other hand, are much 



152. Fibre 



more compact and resistant, and are characterised m y zon 



. . treated 

 ThlS IS 



with 



Muller's fluid. 

 (Schiefferdecker.) 



by their ivory whiteness and opaqueness. 



due to the optical properties of the medullary 



sheath, the finer structure of which has long occupied the 



attention of histologists. 



If a medullated nerve-fibre is examined in the living tissue, 

 or immediately after isolation in an indifferent fluid, it appears 

 as a highly refractive, transparent, and perfectly homogeneous 



