At the junction of the nail-bed and the free border of the nail, the granular and corneous layers again unite with the mucous layer, so that at this point the nail rests upon a complete epidermis instead of on the cells of the mucous layer as in the nail-bed. This junction can be seen through the nail as a yellowish- white line just behind the free border of the nail, if the ball of the finger is pressed against a hard substance. The matrix and nail-bed are freely supplied with arteries and veins which have direct communication without intervening capillaries. This provision permits a temporary stoppage of circulation in these exposed parts without liability of injury to them. The transparency of the nail over the nail-bed allows the color of the blood to show through the nail, while the opacity of that part of the nail covering the matrix (the lunula), due to the changes in the superficial cells of the mucous layer undergoing cornification, entirely shuts off from sight the color of the blood in the vessels of the matrix.
Occasional white spots on the nails are supposed to be due to the presence of small or large air-vesicles between the lamellae or in the intercellular spaces. Unlike the growth of hairs, that of nails is continuous and without definite limit. The growth is supposed to be more active in warm weather than in cold and in youth than in old age. It has been computed that from one hundred to one hundred and sixty days are necessary to reproduce an entire finger-nail and about three times that period for a toe-nail.