The blood vessels of the villi are extreinely numerous.
Small arteries, (a a,fig. 260.) ofabout rolnyth of an inch in diameter, pass between the in testinal tubes. The base of each villus re ceives one, two, or more of these, according to its size. They now pass upwards in the substance of the process, at some dis tance from its surface; and rapidly diminish by giving off numerous capillaries, into which their own trunks entirely merge at about the middle of the height of the villus.
The ultimate capillaries themselves (c c, j7g. 260.) are, on an average, about ird of the above diameter. They constitute a net-work, which lies directly under the basement mem brane ; and covers the whole villus so thickly, as to give it a vivid red colour in injected specimens. The shape and complexity of this network is liable to great variety ; but is usually such, that the length of its meshes is five or six times their width. The capillaries are distinguished by their being apt to ex hibit a wavy and tortuous course (d, fig. 260.) which often :causes their real length greatly to exceed that of the villus itself. This character is especially marked at the free extreniity of the villus:—to the contrac tion of the muscular layer of which it would appear to be chiefly, though not wholly, due.
The veins (v v, jig.260.) come off from this network by the gradual union of capillaries in the upper half of the villus, so as to form two or more venous trunks. These are usually about double the width of the corre sponding arteries : they run at a distance from them ; and often lie rather nearer to the surface of the villus. Belovv, these trunks become confluent in the single vein of the process ; .which, passing vertically downwards, terminates by joining one of the numerous veins belonging to the venous plexus around the orifices of the intestinal tubes. And this latter network also joins that of the villus by such numerous communications, that the two might almost be regarded as merging into each other.
The substance which forms the ground work or basis of the villus resembles, to some extent, that of the gastric mucous membrane ; — the morphological consti tuents of which we have already seen to be indistinct, except at the bottoins of the tubes. It rarely presents any definite struc
ture. Sometimes, however, it is faintly striated. And occasionally this appearance is so marked, as to approach a fibrous character. In this re spect, it resembles the papillm of the skin and tongue ; — and, especially, those secondary projections which stud the fungiform of the latter organ, the basis of which contains no yellow elastic fibres, but is almost homo geneous, and often indistinctly granular.
Mixed with this indistinctly fibrous tissue are numerous delicate cytoblasts or nuclei (b, figs. 259, 261, 262, 263.). The larger of these attain the size of coloured blood-cor puscles ; while the small merge into granules by increasing minuteness. The exact re lation of these to the basis of the villus is un known. Their general effect is to communicate to the whole villus a more or less mottled and granular aspect. This appearance (which we shall find is increased during the period of intestinal digestion) often obscures, not only the vague fibrillation just alluded to, but the whole of the structures which lie beneath the basement membrane.
As regards the lacteals of the villi, few anatomical details have been more disputed than those which relate to the commence ment of the chyliferous absorbents within the substance of these processes. The pro gress of microscopical research has, how ever, reduced the controversy within very, narrow limits ; and promises at no distant date, to end it by a final decision. At pre sent, almost all trustworthy observers agree in the statement, that each villus receives by its base a single (perhaps sometimes a double) branch of the lacteal system. It is only as to the further course of this vessel that opinions differ. Many affirm it to be continued up the villas as a single tube, which ends near its apex by a blind and often some what dilated extremity. Some authorities modify this view for the broader villi, by stating the canal to be double—either as a single loop, or as a bifid and somewhat tor tuous tube. While others find that the cen tral and simple lacteal canal ends by branching into a network of more or less complex cha racter, like that of the capillaries.