222 DR. MICHAEL FOSTER. 



practical utility. If they are excluded, all the little meaning 

 it ever had vanishes. 



It may he urged that we need a word to denote the epi- 

 thelium which is found in the vascular and lymphatic spaces. 

 There does not, however, appear to be sufficient reason why 

 the same term should be applied to the whole of this 

 epithelium. As we have seen, its common mesoblastic origin 

 will not justify this. From a structural point of view, three 

 distinct varieties may be recognised in it, viz. the spindle- 

 shaped cells of the blood-vessels and larger lymphatic vessels, 

 the sinuous cells of the commencing lymphatics and the 

 polygonal cells of the large serous cavities. The fact that 

 the epithelium of the peritonaeum is continuous with that of 

 the lymphatics affords no argument whatever for classing 

 them together. We find continuity everywhere. The epi- 

 dermis is continuous with the alimentary epithelium, and with 

 the urinary and generative epithelium; and the generative epi- 

 thelium is in turn continuous with the peritoneal epithelium. 

 In short, there is no reason why the cells spoken of as form- 

 ing endothelium should have a common title, distinct from 

 the general term epithelium. 



The introduction of the new term is really a step backwards 

 from instead of an advance beyond the old classification 

 adopted by Dr. Sharpey in Quain's ' Elements of Anatomy.' 

 He divides epithelium either physiologically into epidermic, 

 mucous, glandular, vascular, serous, «&:c., or structurally into 

 columnar, spheroidal, ciliated, tesselated, squamous, &c. 



Surely some such nomenclature as this satisfies all require- 

 ments, either morphological or physiological, at least for the 

 present. 



The chief morphological importance, as far as our know- 

 ledge goes, attaches itself to the question from which of the 

 three primary layers any given epithelium is derived, whether 

 from epiblast, hypoblast, or mesoblast; and it is precisely 

 because the phrase endothelium is in this respect misleading 

 that its use is so undesirable. Beyond this, it is difficult to 

 see any morphological interest, unless future research should 

 show that in the common mesoblast there are factors morpho- 

 logically distinct. AVhen that is clearly shown, it will be 

 time to invent new terms which may be as lasting and as 

 valuable as ectoderm and entoderm. 



For physiological purposes all we need is some system of 

 phrases which shall clearly indicate the individual characters 

 and the arrangement of any group of cells. The few terms, 

 ** columnar" or " cylindrical" and " spheroidal," either 

 " ciliated" or " non-ciliated," are almost all we want for 



