94 HYPERTROPHY, HYPERPLASIA, METAPLASIA, REGENERATION. 



example, the epithelium of the nose, bronchi, urinary passages, cervix 

 uteri, aud gall bladder, may under a variety of conditions assume the 

 characters of squamous epithelium of the skin type. 1 



It should be remembered that metaplasia occurs only within the limits 

 of closely related tissues: when differentiation has advanced so that such 

 distinct types of tissue have been formed as connective tissue, epithelium, 

 muscle, nerve, these do not again merge through metaplasia. 



Regeneration. 



General Considerations. It is during the earlier periods of life that 

 the new formation of cells in the body is most active. From the fertil- 

 ization of the ovum until the tissues and organs have assumed the varied 

 forms and functions which the physiological division of labor among the 

 cells imposes, cell proliferation and cell adaptation to a changing envi- 

 ronment are constant and important features of individual development. 

 After this time, under normal conditions, new cell formation is large- 

 ly limited to the replacement of worn-out cells or to the restitution of 

 such cells as may be sacrificed in the performance of their physiological 

 functions. 



The studies of Bizzozero have shown that, notwithstanding the great diversity in 

 the capacity for physiological regeneration among tissues, they may be conveniently 

 grouped into three classes, as follows: First, tissues whose cells are capable of multipli- 

 cation throughout the life of the individual or for a considerable period after maturity, 

 and so lead to a continual regeneration. These are tissues whose cells arc labile and 

 evanescent. In this class are thejiaj^aiiJi^ina.cens of those glands or structures which 



prod uce formed elements, such as the spleen, ly mph -nodeSjJjone-rnarro w , pyary,jsti- 

 cle. Also the epithelium of the skin with its hair follicles and sepacebus glan3sTand of 

 the mucous membranes of the respiratory, digestive, and gcnito-uritiary organs.' 



tissues whose elements increase by division up 

 fora short period thereafter, when evidence of phy 



with p 

 which secrete fluid material, such as the liver, kidnev 



me of birth, or sometimes 



eration "ceases. TT 



cse 



hyma cells of those glands 



Teas, salivarv glands, etc. 

 art'ilaire and bone, and the 



Also members oftlle~coniiec i tive-tissue"~group, fibrous tisSUe, 

 smooth muscle fibres. Tlijr^. striated^muscle and nerve tissue. In thesetissiies. diyi- 

 sion by^ mitosis ceases at an early period and before tlie tissuesThave acquired their spe- 

 ^^cJaTcharacters. Here a physiological regenerationdoes not occur. 



This grouping oftiggues in accordance with their~capacrtT for physiological regen- 

 eration, while liable to modification linden further research, affords a suggestive guide 

 in our studies of regeneration under abnormal conditions. For beyond the regenerative 

 capacity normally exercised by cells in response to the physiological wear and tear of 

 life, they are frequently called upon to make good unusual losses, as the result of many 

 forms of injury. 



Regeneration of injured tissues, all new growths, as well as the 

 hyperplasias above mentioned, are invariably brought about by prolifer- 

 ation or other changes in living cells. Furthermore, just as the cells of 

 the adult organism are the offspring of one original cell, the ovum, so 

 are all the new cells which appear in the body under abnormal condi- 

 tions derived from pre-existing cells by division. 



1 For references to epithelial metaplasia see Menetrier in Bouchard's " Traite de Path, 

 gen.," t. iii., pt. 2, p. 784. 



