63 
The structure of the hair of the mammalia varies greatly in respect to the 
arrangement of the cuticle, cortex and medulla. Wool hairs generally have 
a more or less developed cuticle, but are deficient in medulla, while in 
some ruminants (deer) the medullary portion almost replaces the cortex. In 
rodents the cortical and medullary portion are very distinct, as is well 
known, in the common examples (mouse, rat, rabbit). In the specimens of 
hair, or rather fur, of the mountain hare of the European Alps, and the 
Canadian hare on the table, the construction is as follows ; —the whole hair 
is an elongated spindle shaped flattened tube, running to a fine point at its 
outer end. No imbrication of cuticle scales exists in the shaft of the 
larger hair, the cuticle being a very thin transparent membrane. The 
cortical substance is a simple cylinder, almost colourless. "Within it 
and constituting the chief thickness of the hair, lies a medulla composed of 
rectangular cells so disposed as to form a series of longitudinal and cross 
rows. The walls of these cells are separated by an interspace equal in width 
to about half their own diameter. Viewing this structure by transmitted 
light we see, with proper focussing, a clear exterior border or edge, which 
represents the cortical substance of the enclosing cylinder (this is best seen 
when the hair is split up). Within this border the whole interior is occupied 
by what looks like a rectangular framework of longitudinal and transverse 
bars, having a dark outline ; the spaces between these bars being clear and 
transparent. The clear large spaces between the bars correspond to the 
cells, which when full of colourless fluid are perfectly transparent— the net- 
work of bars, on nearer examination, proves to be the optical expression of 
the contiguous cell walls with air-filled narrow insterspaces. The cell 
wall is composed of horny substance continuous with the internal layer of 
the cortex. With a power of about 220 {\ or ^th) the shaded "bar" distinctly 
shews a double outline of shade with a middle line of clear light -cor- 
responding to the cell walls of contiguous cells, and an interspace which wiD 
appear transparent or not according as it contains fluid or gaseous matter. 
As the hair is widest in its middle portion, and tapers both ways, but 
especially towards its outer extremity, a change in the uniform arrangement 
of the longitudinal bars takes place at different points, as the calibre of the 
cortical tube narrows —instead of running parallel, two bars incline towards 
each other, and uniting at an angle fuse into one. That is to say (interpreting 
the optical expression) a row of cells gradually fines off, and the row on 
either side of it comes into apposition as the middle row disappears. In the 
widest portion of a large hair the transverse row of medullary cells numbers 
4 to 6, in the tapering ends it is only a double or finally a single row. And 
as the hair is & flattened cylinder, the greatest thickness, where the width is 
greatest, corresponds only to two, or at most three cells. 
The hair is therefore a tube of horny substance, within the hollow interior 
