302 DISEASES OF CATTLE. 



an injury and consequent inflammation — as, for instance, proud 

 flesh — there is a limitation of its size, but the controlling influences 

 which govern the size of an organ or normal mass of tissue and limit 

 the extent of an inflammatory overgrowth are all absent in the case 

 of tumors. They are unrestrained, lawless. 



Metastasis expresses the lawlessness of tumors as regards being 

 limited to the original site of development. Small particles of tu- 

 mors enter the blood vessels or lymph streams and are carried to 

 distant parts of the body, where they lodge and start new tumor 

 formations. Expansion by colonization in this manner is a rule with 

 many tumors, and, since they exercise no function of use to the organ- 

 ism, this dissemination of actively growing particles becomes a 

 menace to the system by numerically increasing the body's burden, 

 opening new channels of drain upon the system and adding new 

 centers for the absorption of putrefactive materials when the sec- 

 ondary tumors shall have degenerated. It is this which makes me- 

 tastasis such an important element in the malignancy of tumors. 



Tumors possess no physiological function. They are absolutely 

 useless. Fibrous tumors bind no parts of the organism together; 

 bony tumors add nothing to the supporting framework of the body ; 

 the tissue of fatty tumors never serves as a storehouse of feed and 

 energy; the cells of an adenoma, or gland tumor, furnish no secre- 

 tion ; a tumor composed of muscle tissue produces no increase to the 

 strength of the individual — its muscle cells are not contractile. 



Tumors arise from cells of preexistent tissue. Tumor tissue is not 

 a new variety. Whatever the structure of a tumor, its counter- 

 part is found among the tissues of the body, the lawlessness of 

 the tumor, however, showing itself in more or less departure from 

 the normal type. This departure is usually a reversion to a more 

 elementary or embryonic stage, so that the tumor tissues may be said 

 to be structurally immature. 



Tumors arise without obvious causfe. Concerning the ultimate 

 cause of tumor formation we are absolutely ignorant. Various 

 theories have been advanced from time to time, but none of them 

 have been applicable to more than a limited number of cases. The 

 most important theories may be briefly mentioned. 



(1) The theory of tumnor diathesis. — Bilroth taught that tumors 

 are caused by a peculiar predisposition consisting of a diseased state 

 of the fluids of the body. 'This constitutional taint might be acquired, 

 but, having been acquired, is also hereditary. This theory is known 

 also as the heredity hypothesis, but, while it is true that heredity 

 appears to play some role in the causation of certain neoplasms, its 

 application is too limited to make it of value. 



(2) The Tnechamcal or irntant theory/. — Virchow assumed that 

 tumors arise as the result of previous irritation of the part. This 



