302 ESSENTIAL STRUCTURE OF SECRETING GLANDS. 
they are beset with a series of short tubes opening from 
them, by which the extent of secreting surface is much in¬ 
creased.—On the other hand, although the urinary secretion 
is generally formed by long tubes, yet in the Mollusca it is 
secreted by follicles, according to the general plan of their 
glandular structures. 
359. The secreting cells not unfrequently possess the power 
of elaborating a peculiar colouring matter, either separately, 
or along with the substances which seem more characteristic 
of the secretion. Thus the ink of the Cuttle-fish is in reality 
its urine, charged with a quantity of black matter formed in 
the pigment-cells (resembling those of the interior of the eye, 
§ 533) that line its ink-bag; and the corresponding secretion 
in other Mollusca is rendered purple by the same cause. 
The bile seems to be universally tinged with a yellow or 
greenish colouring matter, which may be regarded, therefore, 
as an essential part of the secretion; and the urine of Mam¬ 
mals is also tinged by a yellow pigment, which seems related 
in its nature to that of the bile. In all these pigments, carbon 
is the predominating ingredient; and their amount is increased 
when the respiratory process is insufficiently performed. 
360. It appears, then, that the different secreting cells have 
the power of elaborating a great variety of products; and that 
no essential differences can be discovered in the structure of 
the glands into whose composition they enter, which can 
account for that variety. We are entirely ignorant, therefore, 
of the reason why one set of cells should secrete biliary matter, 
another urea, another a colouring substance, and so on; but 
we are as ignorant of the reason why, in the parti-coloured 
petal of a flower, the cells of one portion should secrete a red 
substance, whilst those in immediate contact with it form a 
yellow or blue colouring matter; and we know as little of the 
cause, which occasions one set of the cells of which the embryo 
is composed to be converted into muscular tissue, another 
into cartilage, and so on. 
361. One of the most curious points in the Physiology of 
Secretion, is the interchange which sometimes occurs in the 
functions of particular glands. When the operation of some 
one gland is checked or impaired by disease, it not unfrequently 
happens that another gland, or perhaps only a secreting sur¬ 
face, will perform its functions more or less perfectly; this 
