MUCUS. 195 



always containing the same histological elements, it is by no means 

 true that precisely the same secretion is afforded by them all. The 

 ultimate ramifications of the ducts of all true glands (salivary 

 glands, kidneys, &c.) are lined by a mucous membrane^ though 

 their secretions widely differ from each other, and are in no case 

 true mucus. It is more accurate to say that the epithelial cells 

 alone of the membrane secrete. But here again we must exclude 

 the epithelial cells of the minutest gland-ducts in specifying the 

 structure concerned in elaborating mucus. 



Where, however, we find a mucous membrane, in other circum- 

 stances, is expanded as a protective structure (as in the alimentary 

 canal, air-passages, &c.), we also find minute cavities sunk into its 

 substance and opening upon its surface, and the epithelial cells of 

 these are the actual secretors of true mucus. Doubtless, also, the 

 epithelial cells upon the general level of the mucous membrane 

 secrete mucus, but in a less degree. 1 



Mucus may therefore be defined to be the fluid secreted by the 

 epithelial cells of the follicles and of the general surface of mucous 

 membranes, except where they form the lining of minute gland- 

 ducts. 



It is, however, not true that mucus is always, in composition, the 

 same fluid, even when restricted to these limits. It contains an 

 immediate principle called mucosine; but it has already been stated 

 (p. 84) that at least three varieties of this substance exist, to which 

 as many different forms of mucus correspond. Indeed, it is clear 

 that the same fluid would not answer the requirements on the dif- 

 ferent mucous membranes, or on all parts of the same one ; hence 

 the mucus of the mouth is very different from that of the nasal 

 passages, and that of the cervix uteri from that of the uterine ca- 

 vity and of the vagina. A more discriminating investigation of 

 the different fluids termed mucus, because found on mucous mem- 

 branes, is therefore demanded. Physiologically, mucous membrane 

 has no specific character, so far as it is secretive, but only so far as it is 

 protective. 



Mucus is described as a viscid mass, capable of being drawn 

 into threads, and consisting (1) of a pellucid, cohesive fluid, con- 

 taining (2) a variety of morphological elements, principally epi- 

 thelial cells. 



1 Synovial bursce (so called) also belong to this category, and secrete mucus. 



