HISTOLOGY OF NERVE TISSUES 
6l 
conspicuous structures in the preparations are the sheaths of these 
libers, in accordance with which the fibers are classified. 
Medullated Nerve Fibers. —These are the most abundant, 
and possess a double sheath, the outer a thin one of connective 
tissue cells known as the sheath of Schwann or the neurilemma, 
the inner one a thicker fatty layer (blackened by osmic acid) 
known as the medullary or myelin sheath, or the white substance 
of Schwann. The medullary sheath is not continuous, but is 
interrupted at regular intervals, the places of interruption appear¬ 
ing as constricted regions or nodes, and the length of fiber 
between two nodes forming an internode. Note that the sheath 
of Schwann is continuous over the nodes, and that it has one 
nucleus (which often bulges out conspicuously) for each inter¬ 
node of length. Note the double contour of medullated fibers 
in cross section, also the very great range of variation in size 
shown by the cross sectional areas. 
Non-medullated Nerve Fibers. —These are fewer in number 
than the medullated type, and are very inconspicuous. Each 
has a single sheath, the neurilemma, or sheath of Schwann, with 
here and there elongated nuclei which, as they bulge out from the 
side of the fiber, give it a peculiar wavy contour. 
Draw a sufficient length of nerve fiber of each kind y to 
show the above structures. Draw cross sections of medullated 
nerve fibers , showing the two sheaths and a nucleus of the outer 
sheath. 
1. Fresh teased preparation of nerve fibers made by teasing 
upon a dry slide a short length of small nerve taken from a recently 
killed animal (Necturus, frog, or some small mammal). During 
the teasing process the preparation should be kept from too rapid 
drying by breathing gently upon it, and the process of teasing 
should be continued until the separated fibers have dried 
dowm sufficiently to stay in place on the slide. Add a drop of 
physiological salt solution and cover. 
2. Teased preparations of nerve fibers previously hardened, 
and stained in haematoxylin (which stains the axis cylinder, the 
nuclei of the sheath, and the supporting, non-cellular meshwork 
of the medullary sheath), and mounted in Canada balsam (Lab. 
SI. Coll.). 
