148 PATHOLOGY 



The infectious theory of the origin of tumors is also 

 advocated, but the fact remains that the absolute cause 

 of tumors is as yet unknown. 



Structure. The elemental cells of tumors do not differ 

 greatly from normal cells of a like kind, but the relations 

 of the cells to each other and of the tissues composing 

 the tumor are invariably atypic. 



Classification of Tumors. Tumors are sometimes 

 classified according to their embryonic origin, those 

 developing from the middle embryonic layer or meso- 

 blast being termed connective-tissue tumors, while 

 those developing from the hypoblastic or epiblastic 

 layers, being composed of epithelium as well as con- 

 nective tissue, are called epithelial tumors. 



Tumors are also classified, according to their danger 

 to life, as benign and malignant tumors. 



Benign tumors are those which do not spread or recur 

 after removal, do not affect the general health, and are 

 only dangerous secondarily through pressure on vital 

 organs or hemorrhage. 



Malignant tumors affect the general health, usually 

 causing death eventually. They tend to recur after 

 removal and spread to other parts by metastasis. 



CONNECTIVE-TISSUE TUMORS 



Fibroma. A fibroma is a benign tumor composed of 

 fibrous tissue. Springing from pre-existing connective 

 tissue, fibromata are found in numerous localities, the 



