MILK SECRETION. 25 
are able to hold up this milk also, but the majority of cows 
cannot perfectly control the muscle which guards the en- 
trance to the teat. The milk which is present in the milk- 
ducts and which has to pass through these junctions referred 
to above, can be held up by most cows at will. 
All of these small milk-ducts end in small sack-like bodies. 
Each of these dilated portions is called the gland-lobule 
or ultimate follicle. These gland-lobules enclose numerous 
individual microscopical bodies called alveoli or acini. These 
alveoli constitute the organs which possess the proper secre- 
tory functions. These alveoli are lined on the outside with a 
membrane called the tunica propria. Next to this membrane 
is a layer of cell-tissue. The inside layer is composed of cells, 
which are named the epithelial cells. These epithelial cells 
within the alveoli are supplied with blood from the cow’s 
system. During lactation they assume a different form. 
When the cow is yielding milk abundantly, these cells swell 
and extend into the cavity of the alveoli. When the cow is 
not in milk these alveolian cells become flat. A certain number 
of alveoli is tributary to one particular duct leading from the 
gland-lobule into still larger milk-ducts. 
Each aggregation of gland-lobules, tributary to one milk- 
cistern, constitutes a lobe, and may be likened to a side branch 
of a bunch of grapes. Each separate grape may represent 
a gland-lobule. The seeds within the grape, if we imagine each 
seed to be hollowed out and lined with small column-like 
bodies, may be likened to the alveoli. These column-like 
bodies would then represent the epithelial cells. The stem 
leading from each individual grape may represent the small 
duct which carries the milk on to the larger ducts. The main 
stems of the bunch may represent the larger ducts that enter 
into the milk-reservoir. The air which everywhere fills the 
openings or interstices of the various parts of the bunch of 
grapes may be likened to the fibrous fatty tissue between the 
alveoli and the lobules of the gland. 
Theories of Milk Secretion.—Although the theories of milk 
