Henry Gray (18251861). Anatomy of the Human Body. 1918.
which are soft, opaque, granular, and soluble in acetic acid. These are known as prickle cells because of the bridges by which they are connected to one another. They contain fine fibrils which are continuous across the connecting processes with corresponding fibrils in adjacent cells. Between the bridges are fine inter-cellular clefts serving for the passage of lymph, and in these lymph corpuscles or pigment granules may be found.
The stratum granulosum comprises two or three layers of flattened cells which contain granules of eleidin, a substance readily stained by hematoxylin or carmine, and probably an intermediate substance in the formation of keratin. They are supposed to be cells in a transitional stage between the protoplasmic cells of the stratum mucosum and the horny cells of the superficial layers.
The stratum lucidum appears in section as a homogeneous or dimly striated membrane, composed of closely packed cells in which traces of flattened nuclei may be found, and in which minute granules of a substance named keratohyalin are present.
The stratum corneum (horny layer) consists of several layers of horny epithelial scales in which no nuclei are discernible, and which are unaffected by acetic acid, the protoplasm having become changed into horny material or keratin. According to Ranvier they contain granules of a material which has the characteristics of beeswax.
The black color of the skin in the negro, and the tawny color among some of the white races, is due to the presence of pigment in the cells of the epidermis. This pigment is more especially distinct in the cells of the stratum mucosum, and is similar to that found in the cells of the pigmentary layer of the retina. As the cells approach the surface and desiccate, the color becomes partially lost; the disappearance of the pigment from the superficial layers of the epidermis is, however, difficult to explain.
The pigment (melanin) consists of dark brown or black granules of very small size, closely packed together within the cells, but not involving the nucleus.
The main purpose served by the epidermis is that of protection, as the surface is worn away new cells are supplied and thus the true skin, the vessels and nerves which it contains are defended from damage.