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 unfreqnently 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 



