CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 693 



and altered. The cells are manufacturing fatty and other bodies 

 and depositing the products in their own substance, which however 

 is not being renewed but is dying. These changes are still more 

 obvious in the cells lying within the duct ; the cells as indicated 

 by the breaking up of the nuclei are dead, and the whole of the 

 cell-substance has been transformed into the material constituting 

 the secretion of the gland, called sebum, which is discharged on to 

 the surface of the skin through the mouth of the hair follicle. 



In these sebaceous gland secretion, if we may continue to use 

 the word, takes place after a fashion different from that which we 

 have hitherto studied. In an ordinary gland the cells lining the 

 walls of the alveoli manufacture material which they discharge 

 from themselves into the lumen to form the secretion, their own 

 substance being at the same time renewed so that the same cell 

 may continue to manufacture and discharge the secretion for a 

 very prolonged period without being itself destroyed. In a se- 

 baceous gland the work of the cells immediately lining the wall 

 of an alveolus appears limited to the task of increasing by multi- 

 plication. Of the new cells thus formed while some remain to 

 continue the lining and to carry on the work of their predecessors, 

 the rest thrust towards the centre of the alveolus are bodily trans- 

 formed into the material of the secretion, and during the trans- 

 formation are pushed out through the duct by the generation of 

 new cells behind them. The secretion of sebum in fact is a 

 modification of the particular kind of secretion taking place all 

 over the skin, and spoken of as shedding of the skin. It is chiefly 

 the chemical transformation which is different in the two cases. 

 In the skin generally the protoplasmic cell-substance of the 

 Malpighian cells is transformed into keratin, in the sebaceous 

 glands it is transformed into the fatty and other constituents of 

 the sebum. Some perhaps may hesitate to apply the word 

 secretion to such a process as this ; but as we shall see later on, 

 the formation of milk, which certainly deserves to be called a 

 secretion, is a process intermediate between the secretion of saliva 

 and gastric juice and the formation of sebum. 



The so-called 'ceruminous' glands of the external meatus of 

 the ear are essentially SAveat-glands. They are wrongly named, 

 since the fatty material spoken of as ' wax ' of the ear is secreted 

 not by them but by the sebaceous glands belonging to the hairs 

 of the meatus, or by the general epidermic lining. The ceru- 

 minous glands appear at most to supply the pigment which 

 colours the ' wax.' 



The Meibomian glands of the eyelids, on the other hand, are 

 essentially the sebaceous glands of the eyelashes, the glands of 

 Mohl being in turn sweat-glands. 



