28 SECRETION 



but not to the product as a whole. In view of these facts, at- 

 tention will be given in this chapter to several glands which 

 manifestly produce excretions as well as secretions. The 

 action of the kidney and sweat glands is so predominantly 

 excretory that they are treated separately. In what follows 

 the term "secretion" cannot always be taken as meaning a 

 true secretion, for it is customary and convenient to speak 

 of the "secretion of urine," for example. 



Glands. If we conceive of a single layer of secreting epi- 

 thelial cells supported by a thin basement membrane, and 

 then this structure invaginated or folded in upon itself, so 

 that the two layers of epithelium face each other with a 

 greater or less interval between them, with the basement 

 membrane constituting the external support for both, we 

 will have in mind the essential structure of a gland proper. 

 The invaginated cells are the gland cells, and the interval 

 between the two layers of cells is the lumen. Whether the 

 invaginated structure sends off from itself secondary or ter- 

 tiary folds similar to the original, or whether the lumen of 

 any of these folds is in the shape of a simple tube or sac, or 

 both, is immaterial. They may all be considered as identical 

 in nature with the original invagination and only modifica- 

 tions of its architecture. 



However, these modifications are more or less distin- 

 guished by names. Those which become complex by numer- 

 ous branchings of the involuted tube are usually termed 

 compound, as opposed to a single simple fold ; glands are 

 further classified, as tubular, racemose, or tubulo-racemose, 

 according as the termination of the lumen has the shape of a 

 tube, or sac, or both. Thus a simple or a compound gland 

 may belong to any one of the three last-named varieties. 

 The crypts of Lieberkuhn are simple tubular glands. The 

 glands of Brunner are usually described as compound tubulo- 

 racemose structures. 



In a compound gland that portion which communicates 

 with the surface is called the duct and is supposed not to be 



