100 LECTURE III. 



epithelium. A surface upon which cilia were originally 

 seen, may afterwards be found to have ordinary epithe- 

 lium. Thus, on the surface of the ventricles of the brain 

 we meet at first with ciliated, and at a later period with 

 simple scaly, epithelium. Thus, too, we see the mucous 

 membrane of the uterus usually covered with ciliated 

 epithelium, but during pregnancy we find the layer of 

 ciliated cylinders replaced by one of squamous epithe- 

 lium. Thus, also, in places where soft epithelium ordi- 

 narily is found, epidermis may, under particular circum- 

 stances, be generated, as, for example, in the prolapsed 

 vagina. Thus, again, in the sclerotic coat of the eyes 

 of fish, cartilage is found, whilst in man this tunic con- 

 sists of dense connective tissue ; in many animals bone 

 is found in parts of the skin, where in man there is only 

 connective tissue ; but in man, too, in many places 

 where there was original cartilage, osseous tissue is 

 afterwards discovered. But the most striking instances 

 of such substitutions are met with in muscles. One 

 animal has transversely striped muscular fibres in the 

 same place that another has smooth ones. 



In diseased conditions pathological substitutions occur, 

 in which a given tissue is replaced by another ; but even 

 when this new tissue is produced from the previously 

 existing one, the new formation may deviate more or 

 less from the original type. There is therefore a great 

 chasm between physiological and pathological substitu- 

 tion, or at least, between the physiological and certain 

 forms of the pathological one. 



Physiologically, the substitution is constantly effected 

 by the introduction of another tissue of the same group 

 (homology] ; pathologically, very frequently by the 

 agency of a tissue belonging to another (heterology}. To 

 this we must reduce the whole doctrine of the specific 

 elements of pathology which have played so conspicuous 

 a part in the last twenty years. 



