144 ANATOMY FOR NUKSES. [CHAP. XIII. 



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 

 of foreign substances introduced into the body, such as air 

 and food, and also to the contact of secreted matters ; hence 

 their surface is coated over and protected by mucus, a thicker 

 and more sticky fluid than the lymph which moistens the 

 serous membranes. The mucous membranes of different parts 

 are continuous, and they may nearly all be reduced to two great 

 divisions ; namely, the gastro-pneumonic and the genito-urinary. 



The gastro-pneumonic mucous membrane covers the inside 

 of the alimentary canal, the air-passages, and the cavities com- 

 municating with them. It commences at the edges of the lips 

 and nostrils, proceeds through mouth and nose to the throat, 

 and thence is continued throughout the entire length of the 

 alimentary canal to the anus. At its origin and termination 

 it is continuous with the external skin. It also extends 

 throughout the windpipe, bronchial tubes, and air-sacs. From 

 the interior of the nose the membrane may be said to be pro- 

 longed into the lachrymal passages, and under the name of 

 conjunctival membrane, over the fore part of the eyeball and 

 inside of the eyelids, on the edges of which it again meets with 

 the skin. From the upper part of the pharynx a prolongation 



