The observations of Schwarzenberg and Ludwig *, upon dogs in whom intestinal fistul had been carefully instituted, afford much more direct and trustworthy evidence respecting the normal intestinal movements. They introduced into the canal balls of wax, attached to slender lead wires ; and thus verified the following details. The contents of the canal are propelled by a slow continuous peristalsis, which has a definite direction towards the rectum. And although irritation always excites a local contraction, it only gives rise to peristalsis at definite times, during the intervals of which the intestine remains at rest. These times have a general connection with the digestive act: the period of minimum activity being before a meal ; while the maximum of movement is usually from four to six hours after it. But the act of peristalsis is essentially independent of the presence of food; since it may be produced in a starving animal, or in an empty segment of tube. And not only does this intermittent character affect the general occurrence of the propulsive act, but even, to all appearance, its specific phenomena. For when applied at the proper period, a single continuous irri tation produces a repeated and intermittent peristalsis. Hence it is obvious that, during the short intervals of this peristaltic act, the irritation is incapable of exciting contraction.
We may perhaps sum up all these re sults as follows. Direct irritation of this mass of organic muscle excites loca con tractions ; which are of slower access, feebler power, and longer duration, than those of the striped fibre. Shortly after death, these con tractions of the intestinal coat evince a general disposition to extend beyond the site of their origin. But during life, this tendency is so modified by some governing force, that, in obe dience to the requirements of the digestive organ, it is either exalted into a definite and effective peristalsis or altogether suppressed. This definite perista'lsis forms the ordinary mus cular action of the bowel; and is the chief agent in the proper propulsion of its contents. As regards its intensity, we can only conjec ture that it is scarcely more than sufficient to propel the normal contents. As respects its character, it is essentially intermittent. As to its extent, it seems to traverse long segments of the tube. But it remains very doubtful whether every—or indeed any—contraction proceeds continuously throughout the whole length of the intestine. Finally, we have a right to suppose, that at least the more active forms of peristalsis have in them so much of rhythm, as to be not only reeeated, but self repeating, at definite intermissions of time.
But the exact mechanism of this peristalsis remains in obscurity. Nay, more, the infor mation at present at our disposal will not even enable us to take the first step in that process of induction by which alone it will probably be arrived at.
In his admirable Essay on Muscular Move ment, E.Weber* haswell illustrated the peculiar characters of the contractions which are ex cited by the irritation of organic muscle. He has shown that in the Tench (Cyprinus tinca), in whom the muscular coat of the intestine is composed of striped or animal fibre, galvaniz ing the chief nerves of the tube produces an immediate, powerful, and cotemporaneous con traction, in place of the slow, feeble, intermit tent, and enduring action seen in the unstriped or organic intestinal muscle of the other Cy prinoid species. He has also found that the
Iris of various animals repeats the same con trast of structure and irritability. Hence he argues, that the organic muscle is less directly influenced by the nerves; and that these are only connected with this contractile tissue in some such mediate way, as that by which irri tation of the sensitive or afferent nerves gives rise to the reflex movements which are pro ducible in voluntary or striped muscle.
But do these facts warrant such a conclu sion ? On the contrary, do they not render it more probable, that the above varieties of contraction are in some way inherent to the very structure in which these kindred animals differ, rather than that they are brought about by supposed differences of the nervous centres or trunks i—differences (by the way) such as the existing state of our knowledge would rather contradict than establish ? In the intestine of a single Cyprinoid species, the fibre-cell gives place to the striped fibre. Hence, failing all proof of other differences, is it not precisely to this remarkable contrast of structure, that we must refer the parallel contrast vvhich is observed in its contraction, when a stimulus is applied to its nerves ? This direct reference to the structure of the organic muscle seems to be inost justifi able in the case of the local contractions above alluded to ; many of the peculiarities of which are almost what might have been expected from the rudimentary structure, the little vascularity, and (especially) the mode of aggregation, of the fibre-cells. But as re gards the less local contractions of the un striped fibre, their tendency to peristalsis and intermission soon after death, appears to de mand some wider and less continuous connec tion of different points and times, than the tissue itself would directly afford. Such a means of association suggests itself in the ner vous system. And, since the removal of the mesentery does not deprive the contractions of this peristaltic character, any supposition of this kind would appear to refer it to the nerves within the walls of the intestine. But it is difficult to believe that these nerves have ganglia ; nor have any of these essential elements of a nervous centre ever been seen in this situation. While it has been pointed out by Wild, that the excision of a portion of the cesophagus prevents all propagation of its peristalsis beyond the interrupted point :— a fact which tends to show that the contraction of each segment is in some way conditionated by that of its immediate predecessor. The latter experiment, however, supposes such a serious interference with the tube, that any negative result can hardly be regarded as con clusive. And hence, until future researches bring additional information respecting the ultimate distribution of the nerves of this un striped muscular tunic, and the exact arrange ment of its constituent fibre-cells, the relative share of the muscular and nervous tissues in these peculiar contractions can hardly be con jectured. The stimuli by which we excite them in our experiments are in reality far too rude, diffuse, and uncertain in their application, to afford much ground for a decided preference of either muscle or nerve, as forming the chief modulator of that contractility which is, no doubt, essentially inherent to the sarcous sub stance itself.