TUMORS. 
191 
observations show this to be true in a number of sarcomas and 
carcinomas. But not every injury leads to tumors ; if it would, 
not many of our forefathers would have reached a good old age, 
as the mode of bringing up in those days was quite different 
and more forcible than now. Athletics would never have 
reached the present state of development; we would be de¬ 
prived of the pleasure of seeing the pleasant countenances of 
Dr. John Adams and Dr. Sensman if this mechanical theory 
were unimpeachable. 
3. The embryonal or evolutional theory of Cohnheim as¬ 
cribes the formation of tumors to errors of development during 
embryonal life when certain cells having been displaced develop 
the tissue for which they were intended, even if these cells have 
been moved to another locality ; it is the only true way in 
which we can account for the formation of certain congenital 
tumors, as the dermoid casts or certain mixed tumors. It has 
more limited application than the preceding ones. 
4. Nervous theory ascribes the formation of tumors to dis¬ 
tribution in the traphic functions of the nervous system, but is 
more of an imaginary hypothesis than a reality. 
5. Parasitic theory assumes that parasites are responsible for 
the formation of tumors, but it will be hypothetic, as the para¬ 
sites are not demonstrated. 
CLASSIFICATION OF TUMORS. 
1. By nature (which means, whether they are benign or 
malignant). 
2. By shape ; this depends upon the manner of growth, their 
situation and the influence of the surrounding parts. 
3. By structure or hystogenetic classification, which is most 
convenient for a systematic study in the laboratory. 
The classification by nature is into benign and malignant. 
The former do not affect the general health of the patient in 
any notable degree, and may only be dangerous mainly by 
reason of pressure on vital structure of the secondary causes 
(haemorrhage, suppuration, softening), to which they are some¬ 
times liable. Malignant tumors disturb the general health from 
