MUCOUS MEMBRANE. 
Capillaries on the rectum of the Frog. 
a, a, arteries; 6, b, 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 that 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 
rtion of their contour is, however, well de- 
ned 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 
493 
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 Lympuatic 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- 
