300 ESSENTIAL STRUCTURE OF SECRETING ORGANS. 
(fig. 164); and thus the entire mass may he composed of 
numerous lobules, each having its own duct. Passing to still 
higher forms, we find all the ducts coalescing into a common 
trunk, so that the gland bears a strong resemblance to a bunch 
of grapes; as is seen in fig. 165, which represents the structure 
Fig. 165. —Intimate structure of a Composite Gland (the Parotid). 
of the Parotid (one of the salivary glands) of Man. The 
main stalk is the duct into which all the others enter; from 
this pass off several branches, and these again give off smaller 
twigs, the extremities of which 
enter the minute follicles in 
which the secretion is formed. 
These follicles are lined, as in 
their simple condition, with cells, 
which are the essential instru¬ 
ments in the production of the 
secretion; the fluid which they 
separate is poured, by the 
giving-way of their walls, into 
the small canals proceeding 
from the follicles, thence into 
the larger branches, and finally 
into the main trunk, by 
which it is carried into the 
situation where it is to be 
employed or from which it is to pass out. The Liver will 
be seen to possess a structure exactly resembling this, in the 
Fig. 166. —Portion of one of the 
TUBULI URINIFERI OF THE 
Human Kidney; 
Showing its lining of flattened epithe¬ 
lium cells. 
