SIMPLE MEMBRANE. Ill 



tion of cells and of the various tissues ; and, in connection with 

 these, its important vital relations will be particularly indicated. 



Basement Membrane. 



A basement membrane is a mere expansion of simple membrane 

 entering into the structure of the skin, and all mucous and serous 

 membranes, and lying directly upon the corium of these structures 

 respectively. It is itself covered, in turn, by the epithelium of the 

 serous and mucous membranes, and the epidermis of the skin. Its 

 thickness is often not more than 5^33 f an inch. 



Kdlliker does not mention the basement membrane at all as an 

 element of the three tissues just mentioned. There is, however, no 

 doubt of its existence in many parts of them. It has been shown 

 that simple membrane presents three forms ; and Queckett asserts 

 the probability that in one or another of these, basement membrane 

 exists in every part of the three membranes just mentioned. 



Uses. The basement membrane is a complete barrier between 

 the vessels and nerves of the corium on the one hand, and the 

 epithelium on the other, being never perforated by any structures 

 whatever. It thus rises over and covers the villi of the small in- 

 testine, and the papillae of the skin. It also dips down into and 

 lines all the sebaceous follicles and sweat ducts of the latter, and the 

 mucous follicles of the former. Thus it forms everywhere the basis 

 of the epidermis and of epithelium. Whether it secretes from the 

 vessels under it the plasma from which the epithelial cells are de- 

 veloped, is uncertain. This has been supposed to be the fact ; but 

 this supposition is, at least, quite improbable. 



A basement membrane is said by some to exist as a distinct 

 structure in the lining membrane of the bloodvessels also. This 

 assertion still needs confirmation, though the presence of a kind of 

 epithelium there suggests the idea of its presence also from analogy. 

 Kolliker asserts the contrary. 



