286 MANUAL OF HISTOLOGY. 



connective tissue containing blood-vessels, nerves, and lym- 

 phatics. 



The secreting part of the gland (Fig. 122, t) is composed of 

 layers of cells very similar to the cells present in the epidermis, 

 those of the outer part corresponding to the cells of the rete 

 Malpighii. The first layer of cells, viz., those seated upon the 

 basement-membrane, is composed of cylindrical, or cubical, 

 cells, like those of the rete. They have a very distinct nucleus. 

 Further inward the cells become larger, more or less polyhe- 

 dral in form, and contain fat, which obscures or conceals the 

 nucleus. If the fat is extracted the nucleus can be seen lying 

 in the centre of the space previously occupied by the fat. The 

 nearer the centre of the gland the greater the quantity of fat 

 in the cells. The most external layer of cells contains but a 

 small quantity. In the centre of the gland, free fat, fat-crys- 

 tals, and remnants of epithelial cells are found. 



The duct of the sebaceous gland is similar in structure to 

 that of the gland proper. Externally is the basement-mem- 

 brane, lined inside by epidermis-like cells, containing more or 

 less fat, and enclosing a central cavity through which the seba- 

 ceous matter passes to reach the hair-follicle or the free surface. 

 The contents of this canal are fat, fat-crystals, and remnants 

 of epithelial cells. Internal to the polyhedral cells of the duct 

 are the cells of the corneous layer of the epidermis, which di- 

 minish in number in proportion to the distance from the free 

 surface. 



In large hairs the duct of the sebaceous gland opens at an 

 acute angle into the hair-follicle near its upper third, and the 

 gland proper lies about on a level with the middle third of the 

 hair-follicle. 



At the place of union of the hair-follicle with the sebaceous 

 gland the cells of the latter become continuous with the cells 

 of the external root-sheath of the hair. This latter root-sheath 

 becomes continuous above with the cells of the rete Malpighii. 



The development of the sebaceous glands commences at the 

 third month of foetal life, as a projection downward and out- 

 ward of a part of the external root-sheath of the hair, at the 

 place where the future opening of the duct will be situated. 

 It consists, at first, entirely of epithelial cells, which by sub- 

 sequent multiplication and further projection downward, form 

 the sebaceous gland. 



