THE SKIN
THE skin is one of the most active organs of the body It contains the nerve endings of many of the sensory nerves. It is one of the main excretory organs. It plays an important part in the regulation of the body temperature, and it protects the deeper organs from injury and the invasion of micro-organisms.
THE EPIDERMIS
The epidermis is the most superficial part of the skin and is composed of stratified squamous epithelium which varies in thickness in different parts of the body. It is specially thick, for example, on the palms of the hands and soles of the feet. There are no blood vessels in the epidermis, but its deeper layers are supplied with lymph. There are several layers of cells forming the epidermis. They vary in microscopic appearance and are described in two zones.— The horny zone. The germinative zone. The horny zone forms the most superficial layers of cells and consists of:— (a) The stratum eorneum or horny layer.—This is the most superficial layer of cells. The cells have no nuclei and are flat and thin near the surface of the body. The protoplasm in the cells has been changed into a horny substance known as keratin which protects the deeper layers of cells. These cells are continually being cast off by friction and the cells from the deeper layers replace them. The stratum lucidum is composed of cells with clear protoplasm and in some of the cells are found small flattened nuclei. These cells replace the stratum eorneum as it is cast off. The stratum granulosum is composed of several layers of cells which contain many granules in their protoplasm. These granules are thought to be the first stage in the development of keratin. The cells move towards the surface to replace the cells of the stratum lucidum. Distinct nuclei can be seen in the cells. The germinative zone forms the deeper layer of epidermis and consists of:— The prickle cell layer.—This layer consists of cells of different shapes but each cell is characteristic in that it has short thorn-like protoplasmic processes which join them together. Each cell possesses a distinct nucleus. The basal cell layer consists of a layer of columnar-shaped cells with oblong nuclei. These cells reproduce rapidly and are continually replacing the more superficial layers. Within the basal layer there are certain cells known as melanoblasts which contain fine granules of a pigment known as melanin. It is this pigment which gives the colour to the skin over and above that caused by the blood supply to the dermis. The amount of melanin present varies from individual to individual and race to race. In very dark-skinned people the melanoblasts may be present as superficially as the stratum granulosum. Passing through the epidermis are the hair roots, and the ducts of the sweat glands. The surface of the epidermis is ridged due to the presence of raised projections in the dermis. These projections are the papillae of the skin. The ridges formed by the papillae are different in every individual and can be seen clearly at the tips of the fingers and toes. They form the well-known " finger prints ".
THE EPIDERMIS
The epidermis is the most superficial part of the skin and is composed of stratified squamous epithelium which varies in thickness in different parts of the body. It is specially thick, for example, on the palms of the hands and soles of the feet. There are no blood vessels in the epidermis, but its deeper layers are supplied with lymph. There are several layers of cells forming the epidermis. They vary in microscopic appearance and are described in two zones.— The horny zone. The germinative zone. The horny zone forms the most superficial layers of cells and consists of:— (a) The stratum eorneum or horny layer.—This is the most superficial layer of cells. The cells have no nuclei and are flat and thin near the surface of the body. The protoplasm in the cells has been changed into a horny substance known as keratin which protects the deeper layers of cells. These cells are continually being cast off by friction and the cells from the deeper layers replace them. The stratum lucidum is composed of cells with clear protoplasm and in some of the cells are found small flattened nuclei. These cells replace the stratum eorneum as it is cast off. The stratum granulosum is composed of several layers of cells which contain many granules in their protoplasm. These granules are thought to be the first stage in the development of keratin. The cells move towards the surface to replace the cells of the stratum lucidum. Distinct nuclei can be seen in the cells. The germinative zone forms the deeper layer of epidermis and consists of:— The prickle cell layer.—This layer consists of cells of different shapes but each cell is characteristic in that it has short thorn-like protoplasmic processes which join them together. Each cell possesses a distinct nucleus. The basal cell layer consists of a layer of columnar-shaped cells with oblong nuclei. These cells reproduce rapidly and are continually replacing the more superficial layers. Within the basal layer there are certain cells known as melanoblasts which contain fine granules of a pigment known as melanin. It is this pigment which gives the colour to the skin over and above that caused by the blood supply to the dermis. The amount of melanin present varies from individual to individual and race to race. In very dark-skinned people the melanoblasts may be present as superficially as the stratum granulosum. Passing through the epidermis are the hair roots, and the ducts of the sweat glands. The surface of the epidermis is ridged due to the presence of raised projections in the dermis. These projections are the papillae of the skin. The ridges formed by the papillae are different in every individual and can be seen clearly at the tips of the fingers and toes. They form the well-known " finger prints ".