410 „ TUMOURS. 



and benign, when its structure is not widely different from tlie 

 natural tissue, or when it is not an infiltration displacing or over- 

 whelming the original tissue of its seat ; neither does a benign 

 tumour show a natural proneness to ulceration, or if it does ulcer- 

 ate, the ulcer has a tendency to heal, which is not the case in a 

 malignant tumour, where softening precedes an ulceration which 

 has no tendency to heal — a morbid substance like the original 

 forming the walls and boundaries of the ulcer — this substance 

 passing through the same process of ulceration as the primary 

 growth, and so the ulcer spreads and makes it way through all 

 kinds of tissue. 



Tumours are now generally classified upon their histological 

 characters, and are divided into two great groups, namely — 1st, 

 those composed of structures resembling the adult, and 2d, 

 those composed of structures resembling the embryonic con- 

 nective tissue types. The first group is made up of mesoblastic 

 elements, and is arranged as follows : — 



1. — nistioicl or Mesoblastic Tumours. 



(A.) Tumours typical of the fully developed connective 

 tissues : — 



_ papillae of skm or 1 -p, .„ 

 * ^ ^ , C Papilloma, 



mucous membrane, ; 



„ secreting glands, . Adenoma. 

 {C.) Sarcomatous tumours, composed more or less of embryonic 



