Miller expressly says, that in no organ are the free extremities the bloodvessels seen, but that the arteries always pass by a reticulate anastomosis into the veins ; that the blood cir culates between the secreting canals of the liver, and at length on their surface, so as, as it were, to soak their coats with blood, but it does not pass into the canals themselves; or, in other words, that the sanguiferous vessels are not continuous with the hiliary tubes! The important investigations of Kiernan respecting the minute anatomy of the liver, have shewn that the versa ports: having divided so as to constitute an intricate plexus in each lobule of the organ, and having ramified on the secreting canal's, terminates in the hepatic vein.f In the section on the development further evidence is furnished in corroboration of these observations.
In all these instances, then, it is proved that there is no continuity between the arteries and the secreting tubes; and as the smallest secreting canals are always considerably larger than the smallest bloodvessels, the proportion varying in different glands, it may be assumed that in the whole glandular system, the arteries, having divided to a great degree of minuteness, and having ramified freely on the surface of the secreting canals, terminate directly in the re turning veins.
Although in former times such a disposition of the bloodvessels as that now described would have been regarded as incompatible with the process of secretion, yet since the interesting researches of Dutrochet on Endos :nose and Exosmose,t there is no difficulty in understanding that fluids may readily pass from the interior of the arteries into the se creting canals without there being any direct communication between these two orders of tubes. Not only may this passage take place, but even it is rendered probable by the experi ments of Alagendie§ that the bibulous matter constituting the glandular texture, and present ing, as we have found, so many varieties in its physical characters, may separate fluids, varying, according to the gland employed, from the diversified substances mechanically mixed toge ther and suspended in the blood.
Lymphatic vessels.—Notwithstanding these are readily traced in the larger glands, their disposition, and especially their origin, are not known; a connexion, however, has been rather generally admitted in certain glands between their ducts and the lymphatics. In one instance Cruikshank filled the absorbents of the mamma from the lactiferous ducts ; and both Walter and Kiernan contend that the absorbents of the liver may be injected from the biliary ducts. Milner, on the contrary, denies this commu nication, and states that the lymphatics are much larger than the smallest secreting canals.
lle also contends as to the results of injections, that the arguments drawn from them have no greater weight than all others derived from the fortuitous passage by rupture of fluids from one into a different order of vessels.
.Nerves.—In proportion to their size the glands, like the other organs of the vegetative functions, receive very small nerves, which are, with some few exceptions, derived from the system of the great sympathetic. The nervous fibrils surround and accompany the branches of the arteries, till, in the interior of the gland, they become so minute that it appears impossi ble to detect their exact termination. Miller states that they never separate from the blood vessels, and, consequently, that they do not supply the proper glandular substance. But in such a case as this the evidence afforded by microscopical inspection alone should be re ceived with great reserve, especially when it is recollected that in opposition to the doubtful information thus acquired must be placed the unquestioned fact that the mind is capable of influencing the contraction of the secretiog and excreting tubes, as is instanced in the flow of the saliva, of the tears, and of the semen under certain mental impulses. But perhaps a still more striking illustration of the control of the nervous system is afforded by the discharge of several glandular fluids resulting from impres sions acting on comparatively remote but associated surfaces—the pouring forth of the saliva for example, in consequence of the con tact of various substances with the tongue; of the bile and pancreatic juice from the applica tion of food to the surface of the duodenum ; of the semen from the stimulation of the glans penis. In all these and similar instances it must be presumed from analogy that the effect of the physical impression is conveyed through the only known media of conduction, the nerves." The facts here adduced respecting the influence of the nerves merely relate to the contraction of the. secreting and excreting canals ; how far the nervous energy is essential to the process of glandular secretion itself be longs to another division of the subject. (See SECRETION.) Interstitial cellular tissue.—A considerable portion of every gland is made up of the con necting cellular membrane, which, as in all other organs, enters the interior, where it fills up all the minute fissures and angles that intervene between the tubes and lobules, and at length, penetrating between the most minute of the secreting canals, it constitutes. a nidus for the lodgement of the constituent parts.