The first of these statements will at any rate apply to many of the villi. Numerous observers have verified its accuracy for the human subject. And it is not difficult to obtain distinct evidence of its truth in some other Mammalia. Amongst these the sucking Rabbit and Calf are especially suitable for examination. If proper care be taken to examine the chyliferous villi of these ani mals instantly after death, with the aid of suitable fluids, we may easily convince our selves of the presence of a single large lac teal tube, with distinct walls, like that repre sented in the annexed flume (fig. 261.). Such single lacteals are generally very large, having a diameter which often amounts to about one third or one-fourth that of the villus itself; and exhibit a dilated blind extremity, (c, fig. 261.) which nearly doubles their width. In man, according to Frerichs*, they are scarcely more than one-half or two-thirds of this size.
But it remains to be considered whether this statement excludes the possibility of a net work, such as has been affirmed to exist by Krause and others. Koelliker, in whose ad mirable work * the reader will find a copious analysis of the latest observations on this subject, sums them all up very impartially by acknowledging, that, although he has never been able to see a trace of such ramifications, still he cannot venture altogether to deny their existence. On the contrary, he thinks it pos sible that the above simple mode of commence ment,—w hich certainly holds good for the cy lindrical villi,— ma) be exchanged,in the larger of these processes, for one involving the pre sence of a greater number of lacteal canals, or the absence of such blind extremities.
But in conceding this much, Koelliker points out — what Valentin 1- seems previ ously to have suspected, —the facility with which a striated arrangement of the dark fatty molecules within the chyliferous villus may be mistaken for lacteal vessels. Nay more, even the chyle of the central canal sometimes separates by coagulation into strim, which closely imitate a branched network. We may add, that, in the various observations which have been made on executed criminals, the possibility of error has probably been in creased by the distended state of the vascular and lacteal canals contained within the delicate structure of the N illus.
Whatever be the case as regards these conjectures, it seems to me that the large simple tube, and the minute network, are far too unlike to be regarded as mere degrees of development of the same structure in differ ent villi. In like manner, the simple loop of lacteal seen by Henle just beneath the base ment membrane is suspicious, not only from its situation, but also from a fact noticed by Valentin and Remak, —that the central canal sometimes coexists with it. And when we add to the foregoing remarks, that the majority of observers have been unable to see any such ramifications, it will seem difficult to avoid concluding, that each villus probably contains a single large lacteal, which occu pies its centre, and ends by a blind extremity.
The muscular constituent of the villus was first discovered by Bruecke, and has since been verified by Koelliker in many Birds and Mammals. Its shape is that of a thin hollow cone, which closely imitates the form of the %Kills enclosing it. Hence, from whatever side it is examined, it may be seen as a double lon gitudinal layer; which is placed immediately around both sides of the central lacteal ; and lies so deeply within the villus, as to be beneath its vessels, as well as much of its granular basis. It is more distinct in the lower part of the villus, and in the larger flat specimens ; but is easily obscured by oil globules, nuclei, or pig ment. The nuclei of its fibre cells are best seen on the addition of dilute acetic or nitric acid, when they assume their ordinary charac teristic appearance.
The action of this contractile apparatus during life is at present unknown. Derived, as it no doubt is, from that general expanse of unstriped muscular fibre which pervades the whole mucous membrane of the alimentary canal, one can hardly avoid ascribing to it a function which is more or less similar,— if not indeed co-ordinate,—with that of the general stratum. That this function is in some respects related to the static or passive mechanical cir cumstances of the mucous membrane, has already been conjectured (p. 325.) in speaking of the stomach. And the little we know of the ordinary action of the analogous unstriped element in the skin, rather confirms than con tradicts such a supposition. But its pecu liar position with respect to the end of the lacteal trunk in the centre of the vi:lus, — to which it forms a kind of muscular and contractile envelope—has given rise to the suspicion, that it effects the propulsion of the chyle contained in this canal, How far such a process really obtains must be determined by future research, which ought especially to notice the precise con nection of this muscular stratum with that of the mucous membrane generally. In the meantime, we may notice that, as Koelliker justly remarks, an active propulsion by these longitudinal fibres would imply their alternate contraction and relaxation.* But, assuming this to occur,it is evident that such a remittent contraction would not destroy the claims of the absorptive act itself to be considered the chief force which propels the chyle. For in any case, the muscular apparatus would but limit and remove that distention of the lacteal vvhich absorption had previously ef fected. It would thus, as it were, merely re gulate and transfer the mechanical force of the latter act ; so as to modify it, either constantly, or at definite intervals of time.