406 



STRUCTURE OF MUCOUS MEMBRANES. 



hence are readily torn by a moderate force. The redness which they commonly 

 exhibit during life, and retain in greater or less degree in various parts after death, 

 is due to the blood contained in their vessels. The degree of redness is greater in 

 the fo3tus and infant than in the adult. It is greater too in certain situations ; 

 thus, of the different parts of the alimentary canal, it is most marked in the stomach, 

 pharynx, and rectum. 



Structure. A mucous membrane is composed of corium and epithelium. The 

 epithelium covers the surface. The membrane which remains after its removal is 

 named the corium, as in the analogous instance of the true skin. 



The epithelium is the most constant part of a mucous membrane, being 

 continued over certain parts to which the other constituents of the membrane 

 cannot be traced, as over the alveoli of the lungs, and the front of the cornea of 



the eye. It may be scaly 



A B and stratified as in the 



mouth and throat, colum- 

 nar as in the intestine, or 

 ciliated as in the respiratory 

 tract and uterus. When a 

 mucous membrane is co- 

 vered with an epithelium 

 of the scaly and stratified 

 variety, the mucus which 

 moistens its surface is de- 

 rived from glands in the 

 membrane, which are lined 

 with columnar and poly- 

 hedral secreting cells ; but 

 when a columnar epithelium 

 or a ciliated epithelium 

 covers the surface, a large 

 part of the mucus is formed 

 in the cells of this layer, 

 and the glands of the mem- 

 brane are frequently de- 

 voted to the elaboration of 

 some special secretion. The 



secretion is not as a rule formed at the same time in all the cells of the epithelium, 

 but in some only. The first appearance of the mucigen within the cell is in the 

 form of granules ; these appear to become enlarged, and eventually, as they are 

 becoming transformed into mucus, they form clear swollen masses filling a large 

 part of the cell, especially near its free border (fig. 464, m 1 , w?, m 3 ). 



Those cells which are concerned in the production of mucus often become greatly 

 distended with the accumulated mucigen into the sliape of a goblet or chalice, and 

 this may in many be seen to have become exuded from the free and apparently 

 open end of the cell as a droplet of mucus. A certain number of these goblet- 

 or chalice-cells are almost always to be found in columnar epithelium covering 

 mucous membranes. It is somewhat uncertain whether after discharge of their 

 secretion they become reconverted into ordinary epithelium-cells, or whether they 

 permanently maintain the chalice-like form, their cavity becoming again filled with 

 secretion during rest. If the latter is the case the cells in question are analogous 

 to the uni-cellular glands which are met with in the integument of some of the 

 invertebrate animals. 



The corium of a mucous membrane consists of connective tissue, either areolar 



Fig. 464. A. COLUMNAR EPITHELIUM 



FROM THE MUCOUS MEMBRANE OF 

 THE TRACHEA. (E. A. S.) 



m 1 , TO 2 , w 3 , mucus-secreting cells 

 between the ciliated cells, showing 

 three stages in the formation of the 

 secretion. 



B. A MUCUS-SECRETING 

 CELL FROM THE TRA- 

 CHEA J MORE HIGHLY 

 MAGNIFIED. 



