The existence of an irritant of some sort often seems probable, and, although its absence is more frequently determined than its presence, it is obvious that when present it may be overlooked. Although traumatic irritants of considerable mechanical severity exist in but a small percentage of tumors, their occasional influence in the production of morbid growths is not to be denied. Their action may be explained as producing a congestion or as enfeebling the opposition of physiological tissues to pathological growths. The importance of an irritant as the exciting cause, however its action may take place, is supported not only by the sequence of injuries and tumors, but also by the frequent occurrence of tumors in parts exposed to injury and irritation. Such exposure may result from position, structure, or function. The orifices and prominences of the body, the retained testis in the inguinal canal, are notoriously liable seats of tumors. Soft, friable, and slightly resistant structures, like mucous membranes, are not only the frequent place of origin of tumors, but the most exposed parts of such structures are oftenest affected. The exposure resulting from function is manifest by the relation presented by the periods of greatest functional activity of the growth of tumors in such organs as the mammary gland, uterus, and ovaries. The importance of an irritant is still further suggested by the association of tumors with inflammation. The growth of tubercles and cancer from serous membranes is frequently accompanied by an acute inflammation of the latter; fibrous tumors and chronic interstitial inflammations often coexist, while elephantiasis is usually preceded by recurrent, erysipelatous inflammation of the skin. The recent discovery of infective organisms as an exciting cause for many of the members of an entire group of tumors, the granulomata, has resulted in making prominent the etiological rather than the structural features of the tumors concerned. Local peculiarities of tissue, whether congenital or acquired, are thus regarded as representing the beginnings of the growth. With the multiplication of the cells their transformation may take place or a change in their grouping may arise. The essential condition in the production {109} of the morbid growth is that the formation of the cells should take place at an abnormal time or place and should progress in a normal or abnormal manner. The growth takes place with greater or less rapidity in one or another direction according to the nature of the tumor and its seat. The more closely the tumor resembles the normal structures of the body, the slower is its growth; the more it differs in composition, the more rapid is its progress. This difference may arise from a predominance of cells over intercellular substance, as in the case of the sarcoma, or it may result from an atypical combination of tissues, as seen in the development of epithelium and connective tissue in cancer.