342 HISTOLOGY OF THE MORBID PROCESSES. 



but they are characterized as functional hyperplasia or hypertrophy, 

 not as tumor-formation, and are arrested when the needs giving rise 

 to them are met. This limitation of growth does not hold in the 

 case of tumors. 



Our knowledge of the normal forces guiding and restricting the 

 development of the tissues being so deficient, how can we expect to 

 understand the causes underlying the development of tumors ? The 

 marvel is not that certain cells should occasionally continue to mul- 

 tiply and exercise their formative powers without reference to the 

 needs of the whole body. The fact that such occurrences are so 

 rare awaits explanation. Familiarity with what is usual is apt to 

 blind us to the fact that it is not explained, and when our atten- 

 tion is directed to what is unusual we ask an explanation of the ex- 

 ception. A knowledge of the etiology of tumors appears to await 

 the acquisition of a deeper insight into the nature of hereditary 

 transmission and of the conditions which that transmission ordi- 

 narily imposes upon the tissues throughout the life of the individual. 



Tumors arise from the cells of pre-existent tissues. The fact that 

 those cells in producing a tumor form a tissue which is functionally 

 useless is evidence that the usual guiding influences mentioned 

 above no longer completely control their activities. The degree 

 in which that control is lost is, however, by no means the same in 

 all cases of tumor-production. Sometimes the tissues of the tumor 

 attain nearly if not quite the complete structural differentiation pos- 

 sessed by the tissue in which it found origin. In such cases only 

 that degree of normal control which has reference to function 

 appears to be abolished, the cells retaining their special formative 

 activities in nearly full measure and producing a tissue resembling 

 the parent tissue. Such tumors may be regarded as an expression 

 of only a moderate relaxation of the influences normally controlling 

 growth. They are clinically benign. 



While such tumors closely simulating normal tissues are of occa- 

 sional occurrence, in the majority of tumors the formative powers 

 of the cells from which they develop display certain departures from 

 the normal types of the classes to which they belong, and the structure 

 of the tumor becomes different from that of the tissue in which it 

 arose. This departure from the normal formative activity is usually 

 a reversion to a more primitive type of tissue-formation, the control- 

 ling influences normally guiding the cells being weakened to such a 

 degree that the tissues produced fail to acquire the structural differ- 



