Structure of the secreting canals and excre tory ducts.—lt is now certain that all these tubes are composed essentially of a prolonga tion of the mucous membrane. The former, according to Midler, consist only of a single coat, but it must be presumed that they pos sess in addition a tunic, having, independently of elasticity, a power of contraction by which their contents are propelled often in a direction opposed to gravity, and in obedience to the application of a mechanical stimulus to the surface on which the ducts terminate. In the excretory ducts the internal membrane is sur rounded by a fibrous structure, which is very apparent in some of the larger canals, and probably exists in all. The fibres of this coat are of a greyish white or brownish colour, and are often so fine and compact that they are distinguished with great difficulty. The real character of this structure is not known ; in appearance there is little or no resemblance to proper muscle; the action, however, of the excretory canal seems to require a contractile power; and Mcckel states that he has this tioctly perceived circular fibres in the vas deferens, which tube is said to be distinctly muscular in the bull.
Bloodressels.—If it be recollected that the arteries carry to the glands the materials of their various secretions, and if the large quan tity of fluid formed by those viscera be called to mind, we shall not be surprised to find that with a few exceptions, such as the lungs and the brain, there are no organs so abundantly supplied with arterial blood. This supply is in proportion to the activity of secretion, rather than to the size of the gland ; thus the kidneys, furnishing about four pints of urine daily, receive, in proportion to their bulk, more blood than the pancreas, where the secerning process is less active. That this is the principle which regulates the supply of blood is also evidenced in the vessels of the mamma', which receive a more ample supply of blood during lactation than at other periods.
The sanguiferous vessels, like the secreting canals, present many varieties in their dis position in the several glands, the varieties of form in each class being, however, definite in their character, and doubtless having a re ference to the different kinds of fluids which are required to be separated from the circu lating blood. Those organs which are pro vided with a distinct envelope, as the testicle, the kidney, and liver, usually possess but one artery, which enters at the same fissure as the excretory canal ; other glands, presenting a more distinctly lobulated texture and having no proper capsule, the tonsil, pancreas, and mam ma for instance, receive an indefinite number of arteries, which enter irregularly on all parts of the surface; lastly, in the most simple form, as the mucous crypts, the secreting vessels con stitute a delicate plexus on the surface of the little bag.
In all those instances where the gland is large enough to receive one or more arterial trunks, it is found that the vessel loving entered begins to divide into smaller branches, which penetrate between the masses of the gland, and these becoming smaller and smaller at length furnish an intricate plexus, the branches of which, as in the case of the simple bag or follicle, ramify on the surface of the blind secreting canals.
It is only necessary to observe with respect to the veins, that when compared with their arteries, they are smaller than elsewhere; and also that in common with the veins of the splanchnic cavities, they are devoid of valves, so that in the kidney, liver, &c. they may be beautifully displayed by the aid of a suc cessful injection, even to their ultimate rami fications.
Arrangement of the minute bloodvessels.— In considering the intimate texture of the glands, it is essential to state the manner in which the last divisions of the sanguiferous vessels are disposed. By the aid of minute injection these vessels may be demonstrated, though with difficulty, as far as their termina tion; and they may also be observed in a few instances during life and whilst carrying on the circulation.
An opinion to which we shall subsequently recur has been entertained by many anatomists, that the little arteries are either directly con tinuous with the excretory ducts, or, as we should rather call them, the secreting canals, or, at all events, that some kind of direct commu nication exists between the terminal arteries and the secreting canals. The most cautious and apparently successful researches, however, do not corroborate this opinion, but, on the contrary, show that no direct communication of any kind exists. In the lungs, which organs are formed and developed in exact accordance with the glandular structure, the ultimate divi sions of the pulmonary artery, after freely ramifying over the surface of the air-cells, are known to terminate by direct continuity in the radicles of the pulmonary veins. Now, that which is demonstrated in the lungs equally applies in the case of the glands. In the simple laeutim of the mucous membrane the arteries are disposed over the surface of the pouch, but they end in the returning veins without opening on the secreting surface. Miller states that on examining with a suffi cient power the larva of the triton palustris, he observed streams of blood, traversed by single globules, running between the elongated secreting canals of the liver, and, further, that the last arteries pass immediately by a reticu late anastomosis into the small hepatic veins. This disposition is seen in the adjoining figure, which represents the circuit of the blood in the larva of the triton fifteen lines in length.