GLAND. 485 (See the biliary organs of 3telo1outlia l'ulguris, fig. 38, vol. i. p. 111.) In the liver of Alammalia and Alan, both in the embryo and after birth, it is much more difficult to demonstrate the ultimate tubes with their crecal extremities; indeed the existence of such canals is ra ther deduced from the ana logy of the liver in the lower animals than from actual ob servation. Nliiller states that the blind free extremities of the biliary ducts are visible on the surface of the liver with the microscope in the embryo of Mammalia; but that owing to their compact arrangement they are less dis tinct than in birds, so that their internal connexions can not be perceived.* a few Alarnmalia, however, as the squirrel, ( seiurus vulguris,) he observed with the microscope the blind cylindrical extremi ties of the biliary ducts on the surface of the liver, pre senting a branching and foliaceous appearance. ( Fig. 217.) The exact mode of termination of Fig. 217.
the biliary tubuli was still more distinctly seen in a portion of liver considerably magnified, taken from an embryo of the quail, ( Three cuturnix,) about one inch long ( 218).
From the published Fig. 218. account of Mr. Kieman's valuable observations on the minute structure of the liver, it does not appear that the actual terminations of the biliary tubes in blind extremities were perceived, although that such is their disposition is rendered very probable from what was seen with the mi croscope, and especially because it was found that much greater difficulty was experi enced in injecting these tubes than the bloodvessels, on account, as it was sur mised, of the bile contained within them having no exit.* Lastly, in tracing the minute texture of these complex glands of the malia, it is necessary to call the attention of the reader to a circumstance which of all others has been the most fertile source of error, so much so indeed as to have misled the great majority of anatomists. It is this : in many glands small rounded or berry-shaped corpuscles seem to be appended to the commencement of the secreting tubes, so that a deceptive pearance is produced, as if cells or little bags were placed between the terminal bloodvessels and the small excretory ducts. This appearance of cells or even
of solid rounded corpuscles is dent on two causes: in some glands the creting canals are so coiled up that, as is seen in the human testis, when a section is made in the uninjected state an apparently granular ture is presented, ( jig. 219 ;) but a second influential circumstance is that in many stances each of the secreting tubes swells out at its crecal end into a slightly enlarged de-sac ( pedunentated tubes), so that when they are viewed in an aggregate form, the semblance of roundish-shaped granules is seen, (fig. 220.) As these and all other varieties which are presented in the glandular formation arc • L. r. p. 80, § 21, 2'2. Phil. Trans. 18.33, p. 741.
4U lr GLAND.
Fig. 219. considered in the several articles on the individual glands, it is only necessary to state in this place that what are called indifferently lo bules, glandular grains, vesicles, acini, &c. are in every instance composed simply of the secreting canals 'variously disposed and arranged It is, however, very remarkable that whilst those glands which arise from the alimentary canal present an immense variety in the ar rangement of their secreting texture, the essen tial glands of the genito-urinary apparatus, the kidney and the testicle, have a most uniform structure, consisting of serpentine tubes of the same diameter throughout their whole extent.
The details into which I have thought it re quisite to enter prove that the true secreting structure consists in every gland of nothing else than a vascular membrane, on the surface of which the glandular fluid is poured out; and consequently that in those complex organs, as the liver or kidney, in which the 'vascular se Peripheral ramification of the parotid duct, with some of the vesicular terminations, magnified = 110..
Fig. 220.
Otis - N. • • 4104 I I ell .11.