TUMORS. 345 



dencies. The general character of the tissue composing it can be 

 determined ; an approximate idea of the reproductive activity of 

 the cells formed ; the tendency to invade or infiltrate the sur- 

 rounding tissues, and therefore the probability of the occurrence of 

 metastases, estimated ; and the presence of degenerative or other 

 changes observed. The knowledge so gained will throw light upon 

 the clinical significance of the tumor. It is evident, however, that 

 all the knowledge required cannot, in every case, be learned from 

 the examination of a single piece of the tumor. Some of the neces- 

 sary facts are best observed at the periphery of the growth, others 

 in the central portions, and in mixed tumors the various parts of the 

 growth may possess quite different characters. Every tumor must 

 be made the object of a special study, if all the information it is 

 capable of yielding is to be acquired. 



Before passing to a description of the more common types of 

 tumors we must turn our attention for a moment to their classifica- 

 tion and nomenclature. 



Tumors are sometimes grouped in two great divisions : 1, the 

 " malignant tumors," which threaten life because of the rapidity of 

 their growth, their infiltration of surrounding structures, and their 

 liability to metastasis ; and, 2, "benign tumors," which are essentially 

 harmless unless they develop in a situation where they interfere with 

 the function of some vital organ, or unless they appropriate so much 

 of the nutritive material of the body that the general health suffers. 

 This classification is a purely clinical one, and deserves mention only 

 because of its medical importance. There are many degrees of 

 malignancy, and these can be estimated in individual cases only 

 with the aid of deductions from the structural peculiarities of the 

 particular growths. A classification based upon the structure of 

 tumors is, therefore, of greater value than one based merely upon 

 their clinical aspects, for it includes that and much more besides. 



If we bear in mind the fact that any form of cell capable of 

 multiplying may give rise to a tumor, it will become evident 

 that those tumors composed of a single variety of tissue may 

 be classified in a manner similar to that in which the normal 

 tissues are classified. Such tumors are grouped under the term 

 "histioid," to distinguish them from tumors of more complex struct- 

 ure not analogous to simple elementary tissues, which are collec- 

 tively referred to as " organoid." The histioid tumors are desig- 

 nated by names formed from the word indicating the normal 



