TUMORS OF DOMESTIC ANIMALS 6 
THE CONSTITUTIONAL DYSCRASIA OR DIATHESIS THEORY 
The older writers, including Billroth (2), 1 advanced the view that 
a diseased state of the fluids in the body bring about a constitutional 
dyscrasia, which acts as a predisposing cause in the formations of 
neoplasms. This diseased state or predisposition was either acquired 
or inherited. This hypothesis was also advocated by the champions 
of the hereditary origin of neoplasms before the minute structure of 
tissues was known. 
THE MECHANICAL OR IRRITATION THEORY 
With the advent of the cell doctrine, the proliferative activity of 
cells in response to external irritation of a part gave rise to the 
mechanical theory advocated by Yirchow (21) and his followers as 
the cause of neoplasms. Persistent and long irritation of a part 
appears to be a reasonable explanation of some epithelial neoplasms. 
Epithelioma on the lip in pipe smokers, epithelioma of the tongue 
from the irritation of caries of the teeth, carcinoma of the testicle 
in chimney sweeps, and carcinoma and sarcoma of the breast — all 
these have been attributed to irritation resulting from injuries. On 
the other hand the iron bit of a much- abused horse, the ring in the 
muzzle of a bull, the vicious punishment of the pugilist, and the 
violent injuries of the football player are never mentioned as the 
cause of neoplasms. It is therefore possible that injuries and irrita- 
tion may be coincidental with some unknown, underlying cause 
acting at the same time. 
THE " EMBRYONAL RESTS " OR PREFORMATION THEORY 
Cohnheim (If) attributed the development of neoplasm to mis- 
placement of fragments of embryonic tissue in early embryonal life 
before the differentiation of any of the organs was completed. The 
misplaced fragments or embryonal rests lie dormant or sequestered in 
different parts of the body until favorable conditions or physiological 
stimuli bring about their development by subsequent sprouting and 
growth. Xo other theory could account for the presence of hair, 
bone, cartilage, and teeth in the dermoid cysts, or the conglomeration 
of various tissues in the mixed neoplasms of the parotid gland and 
those in the ovary and in the suprarenal gland. Although the theory 
of embryonal rest is valuable to explain these freaks or unusual struc- 
tures in neoplasms, it does not explain the ordinary types of neo- 
plasms. 
Kibbert (17) advanced a modified embryonal theory. He supposes 
that certain changes must precede in the tissue, preparing a favor- 
able soil, before the embryonic rests can start to grow. These changes 
are the product of certain agents elaborated by the tissue surround- 
ing the embr} T onic rests bringing about a " disturbance of tissue 
tension." 
Hauser (&, 9) and his followers maintained the view that the 
changes within the cells of the embryonic rest, rather than the 
changes in the surrounding tissues, must take place to produce a new 
race of cells which are larger in size with a larger nucleus, contain- 
ing more chromatin, and endowed with a greater property for repro- 
1 Numbers in parentheses in italics refer to " Literature cited," p. 39. 
