The size of the capillaries varies much in different parts of the mucous system. In the liver they are very capacious, always one-third wider than the diameter of the blood globule, and sometimes nearly double. In the lungs they are almost equally great. In the intestinal villi also they are of large dimensions. In these organs they form a network on the inner surface of the basement membrane, and are supplied by an artery that ascends in the axis of the villus. The veins from this network are generally two, one on each side. This plexus of the villi is strikingly contrasted by that clothing the tubes that open at their base. In this latter I have observed the diameter to be as small as that of the capillaries of the salivary glands, which do not exceed the width of a blood globule. This disparity is another con firmation of the opinion that the villi are chiefly absorbing, and the tubes secreting organs. Many other varieties might be enumerated, but these are among the most remarkable.
Under most of the compound mucous mem branes bloodvessels are spread out in great pro fusion, and especially in certain localities. The arteries and veins respectively form plane ple xuses, more or less close, more or less intricate, from which emerge branches that pass between the foldings of the simple membrane and com municate with its capillaries, already described. There may even be a series of these arterial and venous plexuses situated one over another, and successively springing out of one another. The effect of this arrangement of an arterial network on one side of the capillaries and a venous net work on the other side, is that the blood, be sides being delayed in their neighbourhood, is most freely and equably distributed in the capillaries themselves : a condition which could scarcely be otherwise accomplished, since, in the case of a villous membrane at least, the capillaries form a series of isolated systems, of which one belongs to each villus. The arranger ment now spoken of exists in the submucous areolar tissue of the stomach and intestinal canal, and in most parts of the skin. In the solid glands, where the capillaries form one continuous system, such arterial and venous networks are not found. At least such inoscu lations, when they exist, are few and rare. In the stomach of many fishes there is a plexus of great thickness under the mucous membrane. In the nose also, chiefly on the spongy bones and septum, there is a plexus of very large veins, well known to anatomists, and also a less capacious arterial plexus ; smaller ones are met with in other parts, as the cheeks and lips, the palate and pharynx. The use of these, especially that of the nose, may be to serve as a diverticulum for the blood in cerebral con gestions. These are the vessels that give way in ordinary epistaxis.
Of the lacteal and lymphatic vessels.—The lacteals have their sole origin from a plexus underlying the simple mucous membrane of the alimentary canal, and it is probable that in every part of the skin a close network exists, such as has been described by several anato mists (see LYMPHATIC Considering the means hitherto at command for ascertaining the precise position of this network, it is not wonderful that disputes should have arisen as to whether it lies in the rete Malpighii, or within the surface of the dermis. I would
hazard the opinion that the real situation of this plexus is underneath the basement mem brane which is everywhere present in the skin.
Of the nerves.—These are numerous and varied, as might be expected from the position of the mucous system in regard to the rest of the body. They may all be styled afferent, and are divisible into three kinds, viz. the sensory, the excito-motory, and the sympathetic. The nerves of special sense distributed to this system are those of smell, taste, and touch. The nerves of common sensation and the excito motory nerves are almost exclusively found here. The tubules of the sympathetic nerves are chiefly given to the proper mucous mem branes and to the glands. All these will be considered more at length under other heads, and they are therefore only referred to here.
Of the °radar tissue.—Before describing the remarkable varieties presented by this tissue under different parts of the mucous system, I must advert to its constitution in those situations where its ordinary characters are well marked—as in subcutaneous fascia, in muscle, on the exterior of the pharynx, &c. Singular as it may appear, there is no correct account of this structure in any of the works on minute anatomy. It in truth consists of two tissues, distinct from each other, and respec tively allied to the white and to the yellow fibrous tissues. The white fibrous element of areolor tissue is chiefly in the form of bands of very unequal thickness, in which are to be seen numerous streaks taking the general direc tion of the whole, but not parallel to the border, nor continuous from end to end. These streaks snore resemble the creases of a longitudinal folding than intervals between separate fibrilla', for which they have been mistaken. These bands split up without difficulty in the long direction, whence result fibrils of the most va ried width, the finest being far too minute for measurement, even with the best instruments.* These bands interlace and cross one another in various directions, and their natural course is wavy. They frequently subdivide and join those near them. Besides these bands, com monly called fasciculi, there are some finer filaments of the utmost tenuity which seem to take an uncertain course among the rest. The yellow fibrous element is everywhere in the form of solitary fibrilla!, which correspond in essential characters with the tissue of that name. They are disposed to curl, and are truly brandied at intervals of variable length ; these branches (which usually retain the size of the fibril from which they spring) becoming continuous with others in the neighbourhood. They have higher refractive properties than the white element, and their borders are conse quently darker.