CHAPTER XXIII. 

 METAPLASIA. 



WHEN a fully developed tissue becomes modified in its structure 

 to resemble another form of adult tissue, without passing through 

 an intermediate stage of indifferent or more embryonic tissue, the 

 process is known as " metaplasia." It differs from the inflammatory 

 process in that the rejuvenescence of the tissue is not obvious, and 

 it is unlike the development of a tumor because the tissue-change 

 is a conversion of one form of tissue into another, and not the pro- 

 duction of a new tissue within another. 



Metaplasia only results in the formation of a tissue closely allied 

 to that in which it takes place. It is most commonly met with in 

 the connective tissues, where a change in the character of the inter- 

 cellular substances and in the form of the cells, which all spring 

 from the same original source, the mesoderm, is all that is necessary 

 to convert one form of connective tissue into another variety of 

 the same group. We must attribute the change to a modification 

 in the functional activity of the cells, the reasons for which are in 

 most cases very obscure. We may, perhaps, in some cases, seek 

 the explanation in conditions that lead to an altered functional 

 demand on the part. Thus, for example, it has been noticed that 

 bone sometimes develops in the fibrous tissues of the thigh or 

 shoulder in soldiers that are obliged to ride or carry a musket for a 

 long time. It may be that the fibrous tissue becomes reinforced in 

 these cases with bone, because it is better calculated to withstand 

 the pressure ; but the fact that such cases are exceptional shows 

 that this response on the part of the tissues is by no means con- 

 stant and that the explanation is incomplete. 



Metaplasia may result in the conversion of fibrous tissue into 

 mucous or osseous tissue ; hyaline cartilage into fibro-cartilage, or 

 into fibrous, mucous, or osseous tissue ; adipose tissue into mucous 

 tissue, etc. The metaplastic tissue is usually not typical ; that is, 

 it differs somewhat from the normally developed tissue in the finer 

 details of its structure. Thus, the bone that is produced by meta- 



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