306 DISEASES OF CATTLE. 



been noticed particularly in the case of certain cancers. They fre- 

 quently develop on the edges of old ulcers, thus being dependent 

 apparently on chronic irritation. Cancer of the lip in pipe smokers 

 is a case in point. Cancerous tumors of the skin often develop on 

 the arms of workers in paraffin, tar, and soot, the chemical irritation 

 of these substances being the cause. On the contrary, the proportion 

 of those thus affected among the exposed is very small and forces the 

 conclusion that if the real cause were in the irritation vastly more 

 cases would occur. 



(3) The theory of nervous influence. — This is based upon (a) the 

 observed fact that tumors occur more frequently in man and the 

 higher animals than in those lower in the scale, among which the 

 nervous sj'stem is less highly developed; (6) that certain formations 

 seem to be directly connected with nerve distribution, while others 

 have been associated with alternations in neighboring nerve trunks. 



(4) The embryonal theory. — This is known also as Cohnheim's 

 hypothesis. In early fetal life there occurs a production of cells in 

 excess of what is required for the construction of the various parts of 

 the body, so that a certain number of them are left over in the fully 

 developed tissue or become misplaced during the sorting of cells for 

 future development of tissues and organs. These cells lie dormant 

 until favorable conditions arise or some sufficient stimulus is applied, 

 when, released from their inactivity, they begin to reproduce and 

 grow. Not being normally related to their site, they lack the control- 

 ling and limiting influences of the part, and, their embryonic character 

 induing them with a most potent proliferating power, they develop 

 in a lawless and unrestrained manner. There are tumors whose exist- 

 ence can be explained only on these grounds. Still, this theory falls 

 far short of answering the question as to the origin of tumors. 



(5) The parasitic theory. — This is not only one of the latest, but, 

 merely as a hypothesis, it is the most attractive and plausible of all. 

 The serious objections to it, however, are the almost uniform failure 

 that has met the attempts to transplant these tumors from one animal to 

 another and the absence of any constant variety of organism in them. 

 Several forms of parasites have been found in certain tumors, but 

 nothing definite has been shown with reference to the relation they 

 bear to the causation of the neoplasm. 



CLASSIFICATION. 



In Senn's work on tumors occurs the following: "A uniform system 

 of classification of tumors is one of the great wants of modern pathol- 

 ogy, and all attempts in this direction have proved failures. " It would 

 be folly, therefore, to burden the pages of a work of this kind with 

 one or several of the proposed systems which have, admittedly, at 

 some important point, failed of their purpose. Since the value of 

 this chapter depends chiefly upon its practical character, which in 



