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use with a well-defined meaning attached to it, it does not 
much matter what its etymology is, or how it is spelt, except on 
historical grounds. Many terms become so altered in their 
meanings, before they finally acquire a permanent application, 
that the chief interest in their etymology is confined to the 
light it throws on the ideas of the man who first introduced 
them. This is the chief reason why new terms should be 
etymologically correct, in order that future inquirers may read 
back through them into the minds of earlier observers. When 
a word is etymologically pure nonsense, this is apt to become 
impossible. Such is the case with “ endothelium.” 
It appears to have been first introduced by His to designate 
the kind of epithelium (“ unachte Epithelien”) which is found 
lining the vascular, lymphatic, and serous cavities of the body, 
in contradistinction to the real epithelium of mucous mem- 
branes (see Die Haute, &e. Le. Akad. Programm. Basel, 1865). 
“Sei es, dass man sie als undchte Epithelien den dchten gegen- 
iiber stellt, sei es dass man sie Endothelien nennt, um mit dem 
Wort ihre Bezeichnung zu den innern Korperflachen auszu- 
driicken.” 
Endothelium is here contrasted with epithel.um, so that 
the latter may be considered as the “thelium” of free surfaces 
(whether invaginated or not), and the former as the thelium of 
internal closed spaces, “thelium” being apparently taken to 
mean “‘a layer,” or “layers of cells.” 
Now what is the derivation of “ epithelium’? Dr Sharpey 
gives the following account: he says, in a letter, “epithelium”, 
or rather “epithelida”, and especially “ epithelia” (1st decl.), 
was first introduced by F. Ruysch. In describing a preparation 
of the face of a child finely injected, he refers to the cuticle 
over the red part of the lip (prolabium), and says, “I cannot 
call this ‘epidermis’, seeing that the subjacent tissue is not 
skin, but a different substance covered with sensitive papille, 
which are finely injected red.” He then goes on to say that as 
