TUMORS. 



"3 



ors composed of embryonic cells extend more repidl)- than those 

 composed of adult cells because embryonic cells are plastic and 

 are to some extent capable of amoeboid movement. Adult cells 

 are fixed in their form and none of them, exceptinjT- leucocytes 

 and endothelial cells are migratory. The nutrient vessels of 

 tumors are sometimes formed of tumor cells that are easil)- de- 

 tachable, a structural peculiarity predisposing to tumor metas- 

 tasis. 



^ 



Fig. 138— Photograph of a section of a horse's lung, showing Metastatic Sarcomata. 



Benign tumors usually extend only by growth in continuity 

 or contigufty. In fact, practically all tumors composed of adult 

 tissues extend bv pushing aside the normal tissue. Malign tum- 

 ors are extended bv blood, as sarcoma; by lymph, as carcin- 

 oma; or they pass from one point to another through natural 

 channels as the digestive and respiratory tracts. 



5-H«r;;;arv.— Tumors may be extended (1) by growth m con- 

 tinuity, (3)" by growth in contiguity, (3) by blood, (1) by lymph, 

 (.5) by 'natural channels other than the blood and lymph vessels. 



Natural Resistance.— Normal tissues have a natural resist- 

 ance to any injurious influence as the formation of tumors, m- 



