HUMAN HAIR CUTICLE 39 The aforementioned tangling process might be thought to be of little consequence for hair on the head where, of course, the root-ends are anchored in the scalp. On the other hand, approximately 100 hairs daily achieve the potential for being shed from the scalp as their follicles reach the resting (telogen) phase of the hair growth cycle. During washing, brushing, and combing of the hair, these are removed quickly, and the brief presence of hairs with detached root-ends seems not normally to give rise to significant tangling of the hair. One might argue that here is a critical proportion of loose hairs in a given hair style below which tangling does not occur but above which tangling might become calamitous. The proportion will increase with increasing time between groom- ings at the rate of approximately 0.1% per day. If, for example, long hair is neither brushed nor combed for several days, this would be expected to increase tangling difficulties during subsequent washing. On rare occasions, such as in those who have lain sick for long periods of time or in those where an unusually high proportion of their hair follicles reach the resting phase (telogen effluvium), the proportion of shed hairs could reach dangerously high levels. This could explain the rare, calamitously tangled, state known as "bird's nest hair" (62,63). HAiR ALIGNMENT If the DFE and free root-ends are responsible for entanglement among mammalian hairs, a corollary is that the same DFE and the absence of free root-ends will result in disentanglement, a suggestion made by Martin (64). Thus hairs on the head (where of course the root-ends are not free to move) predictably will possess a natural tendency to disentangle and to attain parallel alignment with each other. This process is illustrated in Figure 8 in which, according to the DFE and to movements within the hair array, there will be a tendency for the hairs to gain parallel alignment and, interestingly, for the hair array to extend away from the skin surface. Such an effect is clearly of great benefit to man and other animals in maintaining an ordered and extended style for their natural pelage. Systematic scientific study of this effect has not been reported, but one anticipates, as a converse to tangling in loose mammalian fibers, that the magnitude of the DFE as expressed by Equation 1 will determine the extent of disentanglement and of alignment. At any given moment in time, random mechanical events will be responsible for some misalignment between the hairs on the head, and this will be countered by a natural tendency, through the DFE, for the same mechanical events to align the hairs. Thus, in a given hair style, there will be an equilibrium proportion of misaligned (or tangled) hairs, and that proportion will increase as the magnitude of the DFE decreases and vice versa. This conveys the strong message that toiletry treatments of hair that reduce the DFE will cause a greater level of equilibrium tangling in the hair and, heaven forbid that it should happen, a reversal of the DFE would drive the hair to catastrophic entangle- ment (i.e., bird's nest hair). Brushing and combing of hair of moderate length clearly facilitates a shift in the equilibrium to a greater level of ordering. The caveat to this, in the case of very long hair, is that crossed hairs picked up by the tines of the comb tend to be drawn to the ends of the hairs, where cumulative environmental damage will usually have increased the

40 JOURNAL OF COSMETIC SCIENCE Scalp surface Figure 8. Self-alignment or disentanglement of hair. Two types of disorder are illustrated: at left fibers out of parallel alignment, and at right, the tip-end of a hair penetrates the array almost down to the level of the scalp. In both cases, and with mechanical agitation of the array, the directional friction effect results in a preference for the hairs to become aligned with each other (i.e., disentangled) and to extend away from the scalp surface (center). surface frictional resistance. This often results in a tight localized tangle requiring additional force (a snatch) to remove it. REJECTION OF SCALP SURFACE DETRITUS Martin (64) has suggested yet another advantage for the DFE in the rejection of detritus from the skin surface. Particulates of dirt or small insects, having penetrated the mam- malian pelage and reached the skin surface, often cause irritation (the sensation being an itch). The response to this is to scratch or, as is often seen in farm animals, to rub against some solid structure such as a fence post. This serves to mechanically agitate the hairs against the offending detritus. The outcome of these operations is that the DFE of the fibers will be directly responsible for the preferred motion of the detritus away from the skin surface and ejection at the surface of the pelage. This process is illustrated sche- matically in Figure 9.