TUMORS AFFECTING CATTLE. 305 



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 tumors 

 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 organism, this dis- 

 semination 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 secondary tumors shall 

 have degenerated. It is this which makes metastasis such an impor- 

 tant element in the malignancj 7 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 neyer serves as a storehouse of food and energy; 

 the cells of an adenoma, or gland tumor, furnish no secretion; a 

 tumor composed of muscle tissue secures 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 of tissue. Whatever the structure of a tumor, its 

 counterpart 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 maybe said 

 to be structurally immature. 



Tumors arise without obvious cause. 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 tumor diathesis. — Bilroth taught that tumors are 

 due to 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 r61e in the causation of certain neoplasms, its application 

 is too limited to make it of value. 



(2) The mechanical or irritant theory.- — Virchow assumed that 

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



61386—08 20 



