MUCOUS MEMBRANE. 



493 



Fig. 285. 



Capillaries on the rectum of the Frog, 

 a, a, arteries; b, I, veins. 



unusual dimensions, apparently to allow of the 

 more free transit of the blood, which is here 

 propelled feebly by the vis a tergo acting through 

 the capillaries tliat form the portal vein. Though 

 it has not been so described, I believe, from 

 injections that I have made, that the whole 

 organ is one such plexus, and that if it were 

 possible to abstract from it all vessels larger 

 than capillaries, and to leave these entire, all 

 the lobules would still be connected together 

 by capillary channels identical with those of 

 which they themselves principally consist. 

 Hence the lobules of the liver are not definitely 

 bounded on all sides by a capsule of any kind, 

 but here and there blend by continuity of sub- 

 stance with those adjoining them. The larger 

 portion of their contour is, however, well de- 

 fined by the ultimate twigs of the portal vein, 

 and of the ducts derived from the lobule, as so 

 clearly proved by Mr. Kiernan in his well- 

 known paper. 



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 arrange- 

 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 SYSTEM). 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- 



