328 Tumors. 



favorable from irregularities of g'rowth of the individual con- 

 stituents of the tumor (as the imperfect development of blood 

 vessels) or from the absence of ducts for the escape of secre- 

 tions, or from the retention of metabolic products, that regres- 

 sive metamorphoses, as fatty degeneration, calcification, mucoid 

 degeneration, etc., necessarily take place. When such condi- 

 tions prevail and when the structure of the growth is modified by 

 this or that change there is often considerable difficulty in deter- 

 mining the derivation of the tumor and there may be much un- 

 certainty under which group of tumors a given neoplasm should 

 be classed. In such cases we follow the proverb : a potiori fiat 

 deiiominafio ; and the anatomical term is given from the pre- 

 dominating type of tissue present. 



Growth of Tumors. — Derived from one or more tissues, tumors 

 at first form cellular masses (or a tissue complex). These in- 

 crease in size as the internal portions of the new formation 

 multiply, that is, by centra! growth; or they enlarge by progres- 

 sive multiplication of the outer portions, that is, by peripheral 

 or appositional growth. The increase in size and extension of 

 the tumor therefore may take place by one or other of two ways ; 

 either by the tumor growing "as a bulb, lying in the soil and 

 developing, presses aside the ground" (Diirk). "as a rubber bal- 

 loon which one inflates'' (Ribbert) (expansile growth), or by 

 the tumor tissue sending out processes into the neighboring struc- 

 tures as a plant forces its roots into the earth (infiltration growth). 

 It was formerly believed that the enlargement took place by the 

 surrounding tissues becoming gradually involved in the same 

 proliferative process, being actually infected (local infection) ; but 

 this is not probable, as we may usually recognize under the micro- 

 scope that the proliferating cells of the :neoplastic primary focus, 

 that is, the first elements of the tumor, are extending into the 

 surrounding tissue, growing mto the interior of the tissue or 

 toward the surface of the part. The surrounding tissue supplies 

 only the nutrition for growth and takes part only in this sense 

 that its blood vessels and supporting tissue may be appropriated 

 by the new growth, and that it may also multiply as the result 

 of the changes in tissue tension which may be produced (v. 

 chapter on Regeneration) ; otherwise, however, the surrounding 

 tissue plays but a passive part, like the soil in relation to the 

 seed of grain. 



External Shape of Tumors. — In correspondence with their 

 mode of growth autoblastomata form usually circinnscribed 



