28 SECRETION 



not to the product as a whole. In view of these facts, attention 

 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 constitut- 

 ing 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 tertiary 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 invaginat'.on and only 

 modifications of its architecture. 



However, these modifications are more or less distinguished by 

 names. Those which become complex by numerous branch- 

 ings 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 con- 



