CLASSIFICATION 409 



the production of a tumour dissimilar in its structure from that 

 in which it originated, but is applied to a growth dissimilar to 

 that in which it is situated. Thus an excessive proliferation of 

 epithelial cells within the ducts of the mammary gland is 

 homologous, but when this becomes excessive, and extends 

 beyond its normal limits, as seen in cancer, it becomes hetero- 

 logous. 



A tumour rarely actually disappears, and thus differs from an 

 inflammatory growth ; as a rule its permanence is in accordance 

 with the tardiness of its growth. Green says that the more 

 rapid the growth and the more lowly the organisation of the 

 tissues formed, the sooner do retrogressive changes occur. The 

 carcinomata and sarcomata generally develop rapidly, and consist 

 for the most part of cells, which quickly degenerate. 



This cannot, however, be said of rapidly growing tumours in 

 the lower animals. In melanotic sarcoma the growth is some- 

 times very rapid, and shows but little or no tendency to 

 degenerative changes ; but, particularly in lymphadenoma, it will 

 be seen that a most rapid growth is associated with the entire 

 absence of degenerative changes. 



Tumours usually develop from small beginnings. Some grow 

 rapidly, others slowly and intermittently, and there is no limit 

 to their growth, some reaching enormous dimensions. Their 

 formation is always attended with more or less inconvenience 

 and danger. The least injurious are those which grow slowly, 

 and merely compress the surrounding tissues ; but a growing 

 tumour requires to be fed, and the organs on which it may be 

 placed in the system generally are deprived of some nutriment : 

 if the growth be slow this may be unimportant, but if very 

 rapid, the results may be very grave. The gravity and impor- 

 tance of a tumour, benign in itself, are seriously increased by 

 its situation. Thus a very small tumour, particularly if of rapid 

 growth, in the brain or spinal canal may prove fatal to life ; 

 and a tumour pressing upon the sesophagus or trachea, obstruct- 

 ing the acts of deglutition and respiration, is a very different 

 matter to a much larger, invading the skin, and interfering with 

 no important function, 



A tumour is said to be malignant when it has a tendency to 

 destroy and infiltrate into surrounding tissue, when it tends to 

 recur after removal, and when it is metastatic and heterologous ; 



