TUMOURS AND CANCERS. 243 
to dip below the surface and invade the underlying 
tissues. In their simplest form such downgrowths are 
at first solid and club-shaped. Subsequently the central 
cells liquefy whilst the peripheral ones arrange themselves 
ina definite and regular order, so as to forma lining 
membrane to the central chamber, or acinus. The por- 
tion of the acinus near the surface from whence the 
downgrowth originated is slightly narrowed and consti- 
tutes the duct ofthe gland. This is one of the simplest 
F 1G. 122.—The microscopical appearance of an epithelial 
Tumour (Cancer). 
forms of gland, and from it any complex type may be 
derived by buddings or outgrowths from the primary 
epithelial germ (fig. 122). Secreting glands are arranged 
by anatomists into groups according to their structure. 
Physiologists give them specific names according to the 
secretion furnished, such as mucus, sweat, milk, and the 
like. Structurally they form two main groups, tubular 
and racemose. The simple tubular gland is met with in 
the intestine of man and in any mammals, whilst the 
racemose variety is represented by sebaceous glands 
