Barclay Smith speaks in the paper referred to above of the _extensor brevis pollicis_, or _minor_, as a muscle of extremely late appearance, and as “peculiarly human,” and says all the evidence points to its being a segmentation product of the _extensor ossis metacarpi pollicis_, its appearance being foreshadowed in the anthropoid by an extension of that muscle on to the proximal phalanx of the thumb. It is not without interest to the thesis before us to read the rather bewildering story of the early life of a very insignificant muscle such as the small extensor of the thumb of man. As illustrations of the moulding and pruning of perfected muscles it may be remembered that, as Macalister says, “portions of muscles may also become detached and degenerated so as to act as ligaments,” and “the adult muscular system of man bears everywhere traces of earlier cleavings and subsequent fusions, partial disappearances and local outgrowths.”[80] This passage recalls one in which Huxley says in watching certain phases of development you can almost see the hidden artist at work, and here the sculptor may be pictured in his chipping, trimming, rejecting and finally shaping, some creation of his brain; and from a biological point of view a vision of the processes of use and disuse may be obtained. Professor Keith also speaks often of the migrations of muscular attachments in a way which agrees with the passage quoted from Macalister. [80] _Op. cit._, p. 73 CHAPTER XXIII. INNERVATION OF THE HUMAN SKIN. For at least seventy years the surface of the human skin has been the subject of so much physiological observation and experiment that Professor Sherrington considers the literature connected with it to be probably greater than in any other branch of physiology. Most of this study centres round the skin as a receptive field and problems of the nervous system. It is easy to see why this should be in the case of an organ so great as the skin, covering all the other structures and organs and exposed through ages of evolution to the vicissitudes of an inconceivable number of stimuli. And one outcome of this study is to show that, metaphorically speaking, the skin is a mosaic, and not the confused and blurred production of a child of four years old who has been given a sheet of paper and a paint-box. There is order in this field, and even without calling in final causes, plan and purpose. Beside the protective function exercised by the skin it plays a large part, through its nervous endowment, in the processes by which the brain is made aware of the surrounding phenomena, thus conveying intelligence to the centre of life only less important than that of the special senses. It is maintained here that the result of the various physical stimuli, of which pain, cold, warmth and touch are the chief, is that certain functions and structures of the skin have arisen in response to them.