TUMORS AFFECTING CATTLE. 303 



has been noticed particularly in the case of certain cancers. They 

 frequently develoi^ 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, or 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. — That 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 system is less highly developed; (5) 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 those 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 until some sufficient stimulus is 

 applied, when, released from their inactivity, they begin to repro- 

 duce and grow. Not being normally related to their site, they lack 

 the controlling and limiting influences of the part, and, their embry- 

 onic character enduing them with a most potent proliferating 

 power, they develop in a lawless and unrestrained manner. There 

 are tumors whose existence 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 OF TUMORS. 



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

 tem of classification of tumors is one of the great wants of modern 

 pathology, and all attempts in this direction have proved failures." 

 It would be folly, therefore, to burden the pages of a Tvork of this 

 kind with one or several of the proposed systems which have, 



