166 ANATOMY FOE, NUESES. [CHAP. XIV. 



a single layer of epithelial cells, resting usually on a thin mem- 

 brane, on the under surface of which is spread out a close net- 

 work of blood-vessels. In order to economize space and to 

 provide a more extensive secreting surface, the membrane is 

 generally increased by dipping down and forming variously 

 shaped depressions or recesses, these depressions or recesses 

 being called the secreting glands. 



The secreting glands are of two kinds, simple and compound. 

 The simple glands are generally tubular or saccular cavities, the 

 tube in the tubular variety being sometimes so long that it coils 

 upon itself, as in the sweat glands of the skin ; they all open 

 upon the surface by a single duct. In the compound glands, 

 the cavities are subdivided into smaller tubular or saccular 

 cavities, opening by small ducts into the main duct which pours 

 the secretion upon the surface. 



However simple or complicated the involuted surface, the 

 secreting process is essentially the same ; and in this process 

 the nucleated cells play the most important part. These cells 

 take into their interior those substances from the blood which 

 they require to make the special secretion they are set apart 

 to form, converting this selected material into chemical com- 

 pounds, which either act as solvents, as in the digestive juices, 

 or perform some other office in the body. The secretion the 

 cells elaborate escapes from them either by exudation or by the 

 bursting and destruction of the cells themselves. Cells filled 

 with secreting matter may also be detached and carried out 

 entire with the fluid part of the secretion; and, in all cases, 

 new cells speedily take the place of those which have served 

 their office. The glands are provided with lymphatics, and fine 

 nerve fibrils have also been found to terminate in them. That 

 they are under the influence of the nervous system is shown by 

 the fact that impressions made on the nervous system affect 

 the secretions, a familiar instance of which is the flow of saliva 

 into the mouth, caused by the sight, or smell, or even the 

 thought of food. 



The position and functions of the several glands will be de- 

 scribed later in connection with digestion and elimination. 



Mucous membranes. - - The mucous membranes, unlike the 

 serous membranes, line passages and cavities which communi- 

 cate with the exterior. They are all subject to the contact 



