59 
cells of rapid growth, and filled with semifluid matter, in which lies a nucleus 
with a few granules of fat. The medullary cells are continuous with the 
mass of soft cells forming constantly around the papilla at the base of the 
hair bulb. All these cells of the bulb growing older, differentiate either 
into the fibre cells of the cortex, or into the medullary cells. They 
gradually lose their nutritive matter, the cortical fibres retaining 
pigment, and elongated nuclei, but becoming horny. The cells forming the 
central core, at first rounded and full, become dry, angular, and shrivelled, 
their former juicy contents being replaced by fine bubbles of air. The young- 
hair is consequently more translucent, and has greater lustre than old or 
white hair — for its cortex and medulla are composed of cells more homo- 
geneous in composition, and less coloured by pigment, also more evenly 
moistened by nutritive juices, and having a less amount of air included in it s 
medullary cells. As the vigour of the cell-growth at the papilla declines, 
alterations occur, the whole shaft becomes drier, the pigments dissolved in 
fatty media become more granular, the fat more molecular, the membranes 
of the cells thicker, more shrivelled, and less translucent. All such 
changes are accompanied by corresponding changes of optical effect. In 
particular, the reflection of light from the central core of cells is so complete 
when these cells contain air, and their, membrane has lost its smoothness, 
that the medulla appears, when examined by transmitted light, as dark as if 
the cells were filled with black pigment . The effect due to air bubbles in a 
fluid, or crystal, or between laminae of glass, horn, and generally all trans- 
lucent substances, is, as is well known, obstruction to the direct passage of light. 
Thus, light penetrating the surface of a hair, is refracted in its passage to the 
centre, or core, and then sent back again through the cortex if the central 
core be perfectly opaque. Consequently, as the substance of the cortex is 
composed of fibres which are arranged in plates, forming a concentric 
lamination through the whole thickness of cortex, a series of broken 
refractions occur, the proportion of light refracted or reflected at each surface 
varying with the consistence and homogeneity of the laminae, and with the 
state of moisture or dryness of the cell-membranes of which the cortical 
fibres and plates are composed. The reflection consequent on the presence 
of fat or oil molecules, is less than that caused by cells full of air, but fatty 
or oily pigment molecules would cause total reflection, and in proportion to 
the amount of pigment, would the positive colour of the hair predominate. 
Thus the same hair may appear white at one time, and show its natural 
colour at another, according as pigment granules lie in sufficient mass together 
in unaltered parts of the shaft, or as the cortical substance is altered by 
pathological causes. In a hair then, composed of cells arranged in the three 
forms of cuticle, cortex, and medulla, the anatomical disposition of each part 
mainly determines the nature of changes (and consequently of optical effects) 
