58 
opaque. In mammalian hair the cortex is usually fibrous and of horny texture, 
the fibres being built up by the apposition of elongated fusiform cells, con- 
taining dark nuclei and pigment granules. The elongated striation and dotted 
appearance of the shaft of a hair is not solely due to the cell arrangement and 
pigment deposit, but arises in part also from the unequal refraction of light 
through the clear and opaque portions. The normal, fibrous, horny cortex 
does not reflect so much light as would be thrown back by a dense 
granular mass, possessing slight transmitting but strong reflecting property. 
But when oil drops or fat granules accumulate in the cortical substance 
opacity is produced. For reflection of a white tint the fatty chaDge is highly 
favourable. In every tissue, accumulation of fatty molecules occasions 
reflection of light which is white and not coloured, because an aggregation of 
dense refractive particles with interspaces of pigment and horny substance is 
equivalent to an irregidar surface which interrupts and breaks the direct 
reflection of light, whereas a smooth surface reflects, as from a mirror, every 
coloured ray falling on it. Thus a cortical mass composed of numerous 
differently refracting parts breaks up the reflection of colour, and uniformity 
of tint results. Surface reflection producing either a white tint or a true 
mirrored reflection of the outlines and colour of objects may be illustrated by 
comparing the reflection from ground and polished glass (or metal) respectively. 
Refraction through and reflection from the particles or substance of organic 
structures may be illustrated by artificial mixtures of pigments (vegetable, 
animal, or mineral) with water, gums, oils, &c. In the instance of white 
paint, made of partly crystalline or transparent lead particles and oil, a certain 
"body" of white tint results which is not found in mixtures of other white 
metallic or earth oxides. A similar "body" or solidity of white reflection 
is produced by finely granular protein and finely molecular fat mixed, as 
is seen in tissues which have undergone "fatty degeneration." Such 
tissues are very opaque, when examined by transmitted light ; that is to say 
dispersion of light by various refraction in and around the particles increases 
as their transmitting power is diminished. 
On the relative transparency and homogeneity of the cortical substance, 
therefore, depends the amount of light reflected or absorbed into the interior 
of a hair. If the exterior surface of the cortex be simply horny, and not 
dark, pigmented, or rendered strongly reflective or refractive by molecular 
fat deposit, light will penetrate to the inner core of the hair, i. e. the medulla 
(pith,) &c. On the optical properties of the organic elements of this core and 
their particular arrangement in the several types of construction, will depend 
the modifications that occur in the further passage of light. If this core or 
medulla be composed of cells, further reflection and refraction will occur, 
according as the cell wall and contents are more or less translucent or opaque. 
In the youth of a hair, the medullary substance towards the root contains 
