[Illustration: Fig. 39.--Foreleg of domestic dog, showing reversed hair on under surface, which rests on the ground in lying posture.] [Illustration: Fig. 40.--Showing chest of domestic dog, with reversed area of hair on each side.] There are four attitudes adopted by the dog in lying. In the first, when he sleeps he lies stretched out on his side on some surface, with his limbs projected nearly straight out, and in the second, he curls himself up in _his_ armchair in a cosy, rounded posture. But in both these attitudes there is no such sliding pressure as will affect in any way the direction of his hair. In two other favourite attitudes it is far otherwise. When he lies prone he plants his fore limbs out before his chest and either raises his head to the level of his trunk or rests it on his fore paws. Each of these attitudes contributes to a very well-marked change of the hair on the under surface of his fore arms, to use a convenient human term, one which carries us back to the story of man and the apes when their fore arms were discussed. On this surface, from the mechanical conditions involved, a new force, that of _sliding pressure_, comes into play. The skin here is very loose, as indeed it is in the greater part of his body, which may almost be said to form one large subcutaneous bursa. The weight of the fore part of his body and head acts _downwards and forwards_, and thus opposes the normal or downward course of the hair on the limb, such as one sees on the upper surface of his fore arm. The resultant of these two forces has the effect of acting against the normal slope, and a reversed direction of the hair is produced very much like that which is seen in many monkeys and in a small area in man. This is shown in Fig. 39, which appeared in the small book[54], to which reference has been made, and it is confined to the part of the limb where the sliding pressure is seen to act. In this feature again there is a record of his resting habits, and, of course, the time he spends in the fourth attitude with his chin resting on his fore paws contributes its share, the mechanical conditions being similar. [54] _Use-Inheritance._