394 



HISTOLOGY 



between the muscles and the integument on the side of the head, where 

 they form important anatomical features. See the chapter on alimentary 

 tissues of the animals in question (see Chapter XV). 



Sebaceous or Oil-Lubricating Tissues. The second class of lubri- 

 cating tissues is found almost exclusively in the integument of the higher 

 vertebrates (mammals and birds), where there is developed a type of 



gland that is used to produce a fluid 

 for lubrication. This type is essen- 

 tially different from the mucous type 

 in most important respects. It pro- 

 duces an oil instead of mucin. The 

 cell is nearly always destroyed by the 

 act of secretion instead of repeating 

 its duty a number of times. Also the 

 cells that do this always occur in a 

 stratified epithelium in which they can 

 be more conveniently renewed than 

 in a simple layer of cells. The cells 

 that do this work never operate alone 

 but always in some considerable ex- 

 tent of epithelium which is usually 

 invaginated to form a gland. This 

 form of tissue is sometimes secondarily 

 adapted to produce an oil that is at- 

 tractively or offensively odorous (see 

 next part). It is used most com- 

 monly to lubricate the hair in mam- 

 mals, and the feathers in the birds, 

 and our first study may very prop- 

 erly be on the common sebaceous 

 gland of the cat's skin. 



Sebaceous or Lubricating Gland of 

 Mammals; from the Lip of a Cat 

 (Fig. 358). These glands are found 

 clustered around the hairs at the 

 FIG. 35 8. Root of a small hair in the lip P omt where they are about to leave 

 of a cat to show the sebaceous gland the skin and where the sheath is 



(seb.gl.); sh., shaft of hair; ., neck or , . , ... . , , , 



duct of gland in which the secretion col- plainly distinguishable as an mvagi- 

 lects. (Taken from a preparation by nated continuation of the epidermis. 



Being glands that are invaginated 



from this sheath, we may say that they are invaginations of the 

 stratified epithelium. 



The stratified epithelium is carried into the invagination, but is so 



seb. gl. 



