MAMMALIA. 455 



order to increase the secretory surface. Their connection 

 with the epidermis is, however, retained by the ducts which 

 pass outwards, their cavities opening freely to the exterior 

 on the surface of the skin. The great development of skin- 

 glands is a marked feature of the Mammalia, 



We may distinguish two different kinds — (i) the sudorific 

 or sweat-glands and (2) the sebaceous glands. 



1. The sudorific glands are developed by local ingrowth 

 of the basal epithelium of the mucous layer. They lie 

 deep in the dermis and excrete water, with inorganic salts 

 in solution (sweat), discharged freely on to the surface of 

 the skin. The sudorific glands are of the tubular type, 

 coiled and unbranched. 



2. The sebaceous glands are also produced from the 

 basal epithelium of the mucous layer, but are only de- 

 veloped in connection with hair-pits or follicles. Sebaceous 

 glands are usually of the acinous or branching type, and 

 they secrete sebacin^ a fatty substance, the primary function 

 of which is to lubricate the hair. They also differ from the 

 sudorific glands in being necrobiotic^ i.e., the sebacin is pro- 

 duced from dead cells. 



Hair. — A hair is a structure found only in the Mammalia 

 and it can only very doubtfully be compared with feathers 

 or epidermic scales. It is essentially epidermic and its first 

 trace in development is a small process or hair-germ formed 

 from the mucous layer. This protrudes inwards into the 

 dermis and elongates rapidly. Its base then becomes 

 pushed into a pit within which the dermis protrudes, and at 

 the apex of this pit the basal epithelium gives rise by rapid 

 growth to a central axis of cells. The basal pit becomes the 

 dermal papilla and the medullary axis gives rise later to the 

 medulla of the hair. Around the medulla, between it and 

 the basal epithelium, a thin layer or cylinder of the mucous 

 layer becomes cornified, produced above the end of the 

 medulla up to the surface of the corneous layer. Later on 

 this cylinder divides into two so that a cylindrical cavity is 

 produced. This cavity becomes continuous with the 

 exterior and terminates above the papilla. It differentiates 

 the whole follicle into a hair in the centre and the root- 

 sheaths around it. The basal epithelium, next the dermis, 



