TUMORS. 291 



Tissue Types in Tumors. Tumors, as we have seen, are composed of the 

 same types of tissue as those normally existing in the body, and from 

 the latter they are derived by a proliferation of pre-existing cells. Thus 

 tumors of the connective-tissue type originate in connective-tissue cells, 

 of the epithelial-tissue type in epithelium, of muscle in muscle, etc. 



Tumors are not only analogous to the normal tissues of the body in 

 structure, but their life history is manifested under the same general 

 laws of nutrition, growth, reproduction, etc. They are supplied with 

 blood-vessels which grow into them from adjacent healthy parts, just as 

 these do, for example, into granulation tissue, so that they may finally 

 possess a more or less independent vascular system of arteries, capil- 

 laries, and veins. They have lymph-vessels and some of them nerves. * 



While the cell divisions by which tumors grow exhibit in general the 

 same minute phenomena as does cell division in normal tissues (see page 

 95), abnormal mitosis is frequent. Thus it may be multipolar; the 

 chromatic substance may present great irregularity in form and amount 

 (Fig. 30, page 97) ; there may be incomplete nuclear division, or the new 

 nuclei may vary in size and form. While all these abnormalities may 

 occur in the new tissue formations and may be experimentally incited, 

 they are unusually frequent in certain tumors and mark one of the ways 

 in which the lawlessness of tumor growths is manifested. 



Degenerative and Destructive Processes in Tumors. Tumor tissues are 

 subject to the same degenerative changes as other tissues; they may 

 become fatty or calcified, ulcerated, gangrenous, pigmented, etc. By 

 necrosis a tumor may be largely destroyed, though complete obliteration 

 rarely occurs in this way. They are liable to undergo the ordinary in- 

 flammatory changes, granulation tissue may form in them, and abscesses 

 and cicatrices. 



The rapidity of growth of tumors varies greatly ; some grow very 

 slowly indeed and may change but imperceptibly in size and appearance 

 for years, while others grow so fast that they do not acquire solidity, and 

 their elements remain in an incompletely developed condition and are 

 thus more liable to destructive changes than are normal tissues. In 

 healthy tissues the blood-vessels are supported by surrounding elements, 

 which aid them in sustaining the blood pressure from within. In rap- 

 idly growing tumors this external support is often lacking, and, as the 

 walls of the blood-vessels are themselves often badly formed, the result 

 is that the walls are apt to become pouched or aueurismal, and they often 

 burst, giving rise to larger or smaller interstitial hemorrhages. 



Shape, Size, and Modes of Extension of Tumors. Tumors have various 

 shapes: nodular, tuberous, fungoid, polypoid, papillary, dendritic, lobu- 

 lated, etc. (see Figs. 139, 140, 141). 



Tumors may occur singly or in greater or less numbers in the same or 

 in different parts of the body. If they be multiple they may have oc- 

 curred simultaneously or at different times as independent structures. 

 Or, multiple tumors may occur as the result of the dissemination in the 

 body, from a primary tumor, of cells which form a starting-point for 



1 For a study of iierves in tumors see Youny, Jour. Exper. Med., vol. ii., p. 1, 1897, 

 bibliography. 



