io6 BIOLOGY OF THE LABORATORY MOUSE 



and the basal cells together form the stratum germinativum (also called 

 stratum Malpighii or stratum spinosum). The cells of this layer are 

 separated from each other by intercellular spaces. Spines protruding from 

 the cell surface form bridges connecting the cells with each other, and the 

 spines of the basal cells penetrate the connective tissue of the dermis. 



The next three to four layers of cells in the epidermis of the nipple are 

 flattened, have light staining oval nuclei, and possess cytoplasm containing 

 coarse keratohyalin granules. These cells form the stratum granulosum. 

 The intercellular spaces between these cells are considerably narrower, 

 gradually becoming indistinct. 



The next layer consists of about four to five layers of flattened, dead, 

 cornified cells, the peripheral layers of which are constantly in the process of 

 desquamation. This layer is the stratum corneum. 



The surface of the dermis forms projecting elevations, the dermal 

 papillae, which project into the epidermis. The dermis is composed of 

 fibrous connective tissue and adipose cells. It is rich in blood supply. 



Where the epidermis is thin some of the strata described above are not 

 present (Fig. 8i). The cells of the stratum germinativum are cuboidal or 

 polyhedral in shape and form only about one to two cell layers. The stra- 

 tum granulosum is absent. The stratum corneum consists of one to two 

 layers of cornified cells. Dermal papillae are absent. 



Below the dermis loose connective tissue and adipose cells form the sub- 

 cutaneous layer. Where the panniculus carnosus is present, the striated 

 muscle fibers of this layer separate the cutaneous layer from the sub- 

 cutaneous connective tissue. At other places the subcutaneous tissue is a 

 loose continuation of the dermis. 



Pigment is present in the basal cells of the epidermis and in the cells of 

 the dermis of the tail and ear in many difi'erent strains. 



The hair and the vibrissa. — The part of the hair which projects above the 

 skin surface is the hair shaft ; the part within the skin is the root. The root is 

 enclosed in a tubular sac, the hair follicle, which is composed of both the 

 epidermal and the dermal layers of the skin. The dermal part of the 

 folhcle is continuous with the papilla, which projects into the basal part of 

 the hair root. The epithelial cells around the papilla form the hair matrix. 

 These cells multiply, move upward and are responsible for the growth of the 

 hair. The cytoplasm of these cells, in animals which have colored fur, 

 contains pigment. 



As the histological details are essentially similar to those in human hair, 

 for a description of the cell layers of the hair the reader is referred to 



