146 KERATIN AND KERATINIZATION 



of change and in the times likely to be involved (hours or days), with the 

 actual endocrine changes with which epidermal changes are linked. But 

 the argument is general : A and B may be parts of the same organ, SA 

 and SB may be nervous or mechanical signals and T may be very short. 

 Actual situations are never as simple as that of Fig. 60. Usually several 

 cyclic systems interact and at some point a " sensitive " element responsive 

 to the environment may introduce a signal which " gears " the entire 

 system to the diurnal and annual cycles. 



To explain this further it is necessary to introduce another important 

 idea, that of adaptive oscillations. The frequency of the cycle A^±B (Fig. 

 60) is capable of great variation because of variations in the time of 

 maturation of the cells in B (and/or A). Thus when the cycle A ^± B is 

 linked to others C^*D, E^±F, etc., the possibility of coupled oscillations 

 with resonance arises, because the frequencies of the separate systems can 

 change until all have the same frequency (or multiples of this) when they 

 will resonate at this frequency with phase differences determined by the 

 time lags in the various feed-back loops. It is possible to say that the 

 appropriate matching frequencies evolve from the possible range of 

 frequencies by a kind of natural selection — the best-adapted frequency 

 survives and increases its amplitude. This mechanism in essentials was pro- 

 posed by Pringle (1951) in developing a theory of the activity of the brain. 



By such a feed-back train a system of cycles could be " geared " to the 

 diurnal and annual astronomical cycles and an organism's total activity 

 adapted to the physical environment. In these complex events the in- 

 tegument seems, as far as is known, to follow the lead of the endocrine 

 system. It is a target organ of graded sensitivity, but there is little proven 

 evidence of its returning a control stimulus to the deeper tissues. 

 However, this may seem so largely because of our ignorance; the possi- 

 bility certainly exists that epidermal products can be fed back into the 

 organism, thus constituting yet another closed cycle. For example, the 

 grooming habits of both birds and mammals are so persistent that con- 

 siderable quantities of epidermal material must re-enter the organism 

 through the mouth (see p. 59). Further, from the wider viewpoint of the 

 enormous system of communication which constitutes ecology (Hutchin- 

 son, 1948), the integrative function of the integument as a signalling 

 system to predators, to congeners, and to sexual partners can only be 

 mentioned here, emphasized and left. 



Control of epidermal growth 



When we come to consider the epidermis in terms of such general 

 theories, we see at once, as has been emphasized already, that because of 

 its position on the outside of the cell system, it constitutes a special case 

 among the organs. Moreover, it is non-vascular, unevenly enervated 



