. Animal physiology. Physiology, Comparative; Physiology, Comparative. STRUCTURE OF GLANDS. 279 as are those by which the mucous secretions are formed in the highest. But in Insects, the bile is secreted by a small number of long tubes, which open into the in- testinal canal just below the stomach (Fig. 110); and these tubes apparently differ in no respect from those that form the urinary secretion in the same animals, which open nearer the outlet of the intes- tinal canal. In fact, the distinct function of the latter was not known, until it was ascertained that uric acid is to be found in the
. Animal physiology. Physiology, Comparative; Physiology, Comparative. STRUCTURE OF GLANDS. 279 as are those by which the mucous secretions are formed in the highest. But in Insects, the bile is secreted by a small number of long tubes, which open into the in- testinal canal just below the stomach (Fig. 110); and these tubes apparently differ in no respect from those that form the urinary secretion in the same animals, which open nearer the outlet of the intes- tinal canal. In fact, the distinct function of the latter was not known, until it was ascertained that uric acid is to be found in them. In Fig. 160, which re- presents the digestive apparatus of the Cock-chafer, it is seen that the biliary vessels are only four in number, but are very long; and that, for a good part of their length, they are beset with a series of short tubes opening from them, by which the extent of secreting surface is much increased.—On the other hand, although the urinary secretion is gener- ally 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 (§. 533) that line its ink-bag; and the corres- ponding secretion in other Mollusca is rendered purple by the same cause. The bile seems to be universally tinged with a yellow colouring matter, which may be regarded, therefore, as an essential part of the secretion; in some herbivorous quadru- peds it has a green hue, and the colouring matter by which this is given, appears to be identical with that contained in the vegetable tissues on which they Fig. —Digestive Appara- tus of Please note that these images are extr
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