PROPERTIES OF MUCUS. 197 



membrane, covered with epithelial cells. In many regions this compound 

 structure rises into elevations, as in the intestinal villi, or sinks into de- 

 pressions, as in the follicles. 



The epithelial cells are of different kinds, sometimes flat, giving origin 

 to tesselated or pavement epithelium, and sometimes cylin- 



T ., i n - i -i -n , Epithelial cells. 



droid, each cell, in this case, being set vertically upon the 

 basement membrane. In many instances, the cylindroid nucleated cells 

 are furnished upon their outer extremity with vibrating cilia, constituting 

 ciliated cylindroid epithelium. Both forms of epithelium, the tesselated 

 and the cylindroid, coexist in glandular ducts. The origin of the cells 

 is in the basement membrane, from germs arising there ; and as the older 

 and therefore superficial cells exuviate or deliquesce, new ones arise to 

 take their places. 



After what has been said, it is not necessary to give a detailed de- 

 scription of mucous surfaces farther than to state that from properties 

 them there is furnished a viscid, glairy fluid, of different shades * of mucus, 

 of color from white to yellow, denser than water, and insoluble therein. 

 Examined by the microscope, it contains granular corpuscles and epithe- 

 lial cells. Its reaction is alkaline, and its proximate constituent is a sub- 

 stance to which the name of mucin has been given. Derived from dif- 

 ferent sources, as the nasal, bronchial, and pulmonary surfaces, the in- 

 testinal canal, and the urinary and gall bladders, it exhibits specific dif- 

 ferences. Its quantity is often greatly increased by morbid causes, as, 

 for example, in catarrh, its composition likewise varying at different 

 stages of the same disease. Its use, for the most part, seems to be the 

 protection of the delicate structure which secretes it. In some positions, 

 as in the intestinal canal, it likewise probably acts in the way of reliev- 

 ing friction of the substances passing over surfaces. 



Of secreting Glands. The typical form of secreting cell-gland is a 

 single cell, with its nucleus at the lower end, the other end simple sac-like 

 having become open by deliquescence or dehiscence, and thus cell-gland, 

 constituting a sac. From the nucleus thus situated at the end of the 

 cavity broods of young cells arise. These become more perfect as they 

 advance toward the mouth of the sac. The outer wall, and especially 

 the region of the nucleus, is furnished copiously with blood-vessels. 



Of such structures, variously modified, the different glands are com- 

 posed. We shall now proceed to the description of the more important 

 of these, as the liver, kidneys, mammary gland, &c., again impressing 

 the remark that, though all these glands are the seats of myriads of cells, 

 cell life is for increased organization, and secretion is in many instances 

 nothing more than filtration or strainage. We shall endeavor, as the 

 occasion arises, to show, in the case of each gland, what part of its action 

 is due to cell influence, and what to such mechanical permeation. 



