61 
absence of protecting grease. Then the separation of the horny plates of the 
cortex, and the entrance of fine air bubbles between the cells or concentric 
laminae might be considered possible, and quite adequate to prevent the 
colour of the pigment from being seen so well as before ; as also to produce 
strong reflection of light. Lastly, the medullary cells, by suddenly filling 
with air, would develope an equally sudden whiteness by refraction of 
light, supposing the horny substance of the cortex to be moderately trans- 
lucent. 
Thus whiteness, having a certain body to it, would be more noticeable in 
a hair dried and split, or cloven into fissures and planes, into which air had 
insinuated itself, than in a hair from which pigment had been removed, and 
which would simply be colourless and more or less transparent. It is not 
improbable that the few instances in which sudden whiteness of hair has been 
recorded as the effect of mental disquiet, are due, in reality, to previous un- 
suspected changes of texture and organic deposits, rendering the hair liable 
to a rapid alteration of its structure and optical properties. Extreme fright 
and other depressing mental emotions afflict countless millions without affect - 
ing their hair. Moreover, the probability that only rapidly decaying hair is 
subject to such changes, is rendered almost a certainty when we consider 
that the adult well-pigmented hair withstands all outward influences in a 
remarkable manner, and is amongst the most indestructible parts of the body. 
The gradual process of change, from lustrous coloured dark to dull white, is 
well known as being due to disintegration of the molecular pigment and fatty 
deposit. Clear grey and translucent white gradations are due to the partial 
preservation of the translucent character of the cortex and medulla with 
relatively small proportion of pigment. With alteration of the physical pro- 
perties of the cell membranes and contents, a corresponding change of optical 
effect must result. The dark colour of the skin in negroes depends on pigment 
deposit in the epidermis (homologous with hair cortex.) The pigmented skin 
of the negro never turns white from fright. 
The occurrence of pathological whitening of the human hair, here touched 
upon in answer to some observations made during the discussion, is altogether 
beside the question of the normal growth of white hair in animals inhabiting 
mountainous and polar regions. A moment's consideration will show that 
the whitening of human hair, whether by a natural process of degradation 
of organic particles, or by a sudden derangement of texture, can have little 
relation to the formation of the winter fur of an animal— a process governed 
by laws of growth and differentiation of tissues. The optical effects, how- 
ever, resulting from structural disposition of elements, possessing (singly and 
in certain combinations) characteristic physical properties, are the same 
whether studied from one or other point of view, and justify, so far, this 
digression. 
