CHAPTER IX. 



TUMORS. 



The Nature and Characters of Tumors in General. 



THE word tumor, which was originally applied to any swelling, has 

 become more and more strictly limited in meaning until it is now com- 

 monly used to designate certain more or less circumscribed new tissue 

 growths or neoplasms. But the formation of new cells and new tissues 

 is of such common occurrence under normal as well as under various ab- 

 normal conditions that it is desirable for us to fix if possible upon some 

 features by which the tissue growths called tumors may be distinguished 

 from other normal or abnormal new-formed tissues. In the first place 

 tumors, like all tissues formed in the body after embryonic life, are pro- 

 duced by the proliferation of cells of normal types, each of its kind. 



It would be well in this connection to remember that a given cell of 

 whatever type, be it of connective tissue, muscle, gland, or nerve, can 

 fully maintain the characters of its type only so long as the conditions 

 of its life do not greatly differ from those under which by adaptation to 

 environment the type was finally fixed. The characters of cells are 

 largely shaped by heredity, and these characters are maintained with 

 great tenacity, but both in form and function cells may be largely swayed 

 by external influences. The marks of departure from the type in cells 

 subjected to an unusual environment may be evident when expressed in 

 form or structure, and sometimes in function, but it is well not to forget 

 that even our most refined methods of study of cells leave by far the 

 greater part of their metabolic performances wholly in the dark, while 

 we seem as yet to be only 011 the threshold of knowledge of their structu- 

 ral details. 



These considerations are especially important in the study of tumors 

 as of all new tissue formations, because the key to our understanding of 

 these lies largely in the appreciation of the fact that every cell in the 

 differentiated tissues comes from a cell of the same type. But this, 

 which is sometimes called the principle of legitimate succession in cells, 

 assumes that the conditions under which the life of the new cell is main- 

 tained shall not depart too widely from the normal, that is, the'usual. 

 The bearing of this limitation will be evident as we proceed in our study 

 of tumors. 



The capacity for cell proliferation is most marked in the embryo and 

 during the earlier periods of individual life. When the body attains its 

 development, the power of cell reproduction, as we have seen see Re- 

 generation, p. 94 is largely limited to special forms of tissues, such as 

 connective tissue, the blood, and certain kinds of epithelium. This 

 19 



