of Edinburgh, Session 1885 - 86 . 
661 
frequently angular in shape, and relatively of a larger size than the 
cells forming the cortical substance. The medulla always occupies 
the centre of the shaft, hut the diameter of this portion relative to 
that of the hair varies very much in different hairs. The medulla 
seldom exists continuously throughout the whole length of the 
hair, being unusually absent at the point and interrupted at parts for 
a greater or less extent. In the latter case, the axis of the stem at 
the interruptions may be fibrous, like the surrounding cortical part, 
or these intervals may be occupied by a granular or a clear colour- 
less matter. In some cases the medulla presents the appearance of 
a canal running along the axis, and filled with a more or less trans- 
parent homogeneous substance or a coloured or white opaque granular 
matter. In both cases there are frequently considerable air cavities. 
Variations in the structure of the hair may occur in any of the 
parts of which it is composed, and those who know how very wide 
this variation is in the different hairs found in the animal kingdom, 
will not be surprised at its extensive occurrence within the narrower 
limits of any of the species. 
The hair or wool of the sheep ( Ouis aries ) has a distinct indivi- 
duality of its own, when in its highest state of development, but the 
the tendency to run into varieties, which is so marked in this class 
of animals, is also true of the fibrous covering. Purity of breed and 
cultivation tend to check variation in every direction, and the 
greatest uniformity in the likeness of individual fibres is always 
found in the most cultivated sheep, such as the merinos ( Ovis 
Hispaniam ), where the departure from the structure of true hair, as 
distinguished from true wool, is most marked. A classification of 
wool may indeed be based upon the variation from the structure of 
hair, which will embrace all the varieties of wool which are found 
either in wild or domestic sheep. To understand this classifica- 
tion, we must notice the difference in structure between the widest 
extremes of hair and wool. 
In hair the scales of its cuticle are firmly attached to the cortical 
substance throughout the greater part of their length, and only 
reveal themselves when examined under the microscope as fine 
irregular transverse and anastomosing lines on the surface and by a 
slight denticulation at the edge of the hair when viewed by trans- 
mitted light. A typical illustration of this surface structure may be 
