The A.B.-Z. of our own nutrition

by Horace Fletcher

A remarkable feature in the segmentation of the food is the rapidity with which the changes take place. The simplest way of estimating the rate of division is to count, not the number of times the partition of the food recurs in the same place, but the number of different sets of segments observed in a given period. Thus in Figure 4 the appearances of lines 1, 2, 3, 4, etc., would be counted, and not merely lines 1, 4, etc. Repeated observations on different animals have shown that the most common rate of division in long, thin chains of food varies between twenty-eight and thirty times in a minute; i. e., there is a change from one set of segments to another set every two seconds, and a return of the same phase every four seconds. In some cases the rate is as low as twenty-three times per minute. The larger masses seem to be associated with a slower segmentation; the operations indicated in Figure 3, for example, occurred from eighteen to twenty-one times in a minute, so that the same phase reappeared only once in eight or nine seconds. The segmentation frequently continues for more than half an hour; in one instance it was seen to persist with only three short periods of inactivity for two hours and twenty-two minutes. At the rate of thirty segmentations per minute it is clear that a slender string of food may commonly undergo division into small particles more than a thousand times while scarcely changing its position in the intestine.

I have seen once, in a cat only lightly etherised, the exterior of an intestine which was dividing the food as above described. An hour and a half after a meal of salmon the anæsthetic was given, the abdomen opened, and the flaps raised so as to form walls. Warm salt solution was then poured into the abdominal cavity, and the floating coils left covered with the transparent omentum. The gastric peristaltic waves were running regularly; on the intestine there were visible at various places during the period of observation regions of constriction which had the appearance shown in Figure 3, except that the rings were relatively nearer together. New rings of constriction took place on the same side of all the bulging parts at the margin of the constricted portion (cf. dotted lines, Fig. 3). As new rings occurred the old relaxed, but apparently with tardiness, for the contents gurgled as if forced through the narrowed lumen. The constrictions recurred irregularly and at much longer intervals than in the normal animal. The contracted rings were pale and bloodless.

The effect of the process of rhythmic segmentation proves it an admirable mechanism. The food over and over again is brought into closest contact with the intestinal walls by the swift kneading movement of the muscles. Thereby not only is the undigested food intimately mixed with the digestive juices, but the digested food is thoroughly exposed to the organs of absorption. Mall has shown that contraction of the intestinal wall has the effect of pumping the blood from the submucous venous plexus into the radicles of the superior mesenteric vein, and thus materially aids the intestinal circulation. Moreover, lacteals loaded with fat will in a few moments become empty unless the intestine is slit lengthwise, so that the muscles cannot exert compression. The rhythmic constrictions, therefore, both propel the blood in the portal circulation and act like a heart in promoting the flow of lymph in the lacteals. This single movement with its several results is an excellent example of bodily economy; the repeated constrictions, as already shown, thoroughly churn the food and digestive fluids together, and also plunge the absorbing mucosa into the very midst of the food masses: but not only are the processes of digestion and absorption favoured by these movements; they also, by compression of the veins and lacteals of the intestinal wall, serve to deport through blood and lymph channels the digested and absorbed material.

Peristalsis.[34]—The phenomena of peristalsis and segmentation are usually combined in some manner while the food passes through the small intestine. Peristalsis is observed normally in two forms: as a slow advancing of the food for a short distance in a coil, and as a rapid movement sweeping the food without pause through several turns of the gut. The latter form is frequently seen when the food is carried on from the duodenum; and it may readily be produced in other parts of the small intestine by giving an enema of soapsuds.

Figure 4.—Diagram showing combined peristalsis and segmentation.

When a mass of food has been subjected for some time to the segmenting activity of the intestine, the separate segments, instead of being again divided, may suddenly begin to move slowly along the loop in which they lie. That this movement is not a swinging of the coil as a whole, but a peristaltic advance of separate rings of its circular musculature, is made probable by the fact that the succeeding segments follow along the same path their predecessors have taken. The advance of the little pieces may continue for seven or eight centimetres, when finally the front piece stops or meets other food. Then all the succeeding pieces are swept one by one into the accumulating mass, which at last lies stretched along the intestine, a solid string manifesting no sign of commotion.

Another form of slow peristalsis is frequently observed when the food is pushed forward, not in small divisions, but as a large lump. The relatively long string of food is first crowded into an ovoid form as the forward movement begins, and as it is collecting thus, it seems at the last to be suddenly formed into a more rounded ball, as if the mass were pulled or pushed together at the two ends. The next moment it is indented in the middle by a circular constriction (as shown in Fig. 4, line 2), which spreads it in both directions along the loop. The trailing portion (a) is next cut in two, and the severed part sometimes flies back over its course about a centimetre. Now the whole mass is swept together again and slightly forward as shown in line 4, Fig. 4, and the segmenting process is repeated. At stage 3, Fig. 4, a constriction sometimes appears around the middle of the advanced portion (b). Thus, with many halts and interruptions, the food slowly advances.

A slight variation of the movement just described is observed when the amount of food is greater and extends farther along the intestine. Under such circumstances, as the mass moves forward, constrictions appear just in front of the rear end, which separate it from the main body, and cause it to shoot backward sometimes through the distance of a centimetre. The main body meanwhile is not disturbed. No sooner has the rear section been shot away than it is swept forward again into union with the rest of the food, and the whole mass then advances until another interfering constriction repeats the process.

Rhythmic segmentation and the pendulum movement.—There is little doubt that the segmentation of the food which I have seen is due to an activity of the intestinal musculature similar to that which causes the so-called pendulum movement. This activity, as already noted, is rhythmic, and, although accounts differ, analytical methods prove that it involves both the longitudinal and the circular layers of muscle. Observations of the effect of the rhythmic contractions upon the food show that the action of the circular fibres is most prominent. It is probable, however, that the longitudinal fibres also play an important part in the process of segmentation. Examination of Figure 2 makes clear that in line 2 the regions of constriction appear between the regions of constriction in line 3; before c can be formed, therefore, the constriction between a and b must relax. Contraction of the longitudinal fibres between two segments would help to enlarge the constricted lumen of the gut. It seems probable that, as the constrictions on either side of c occur, the longitudinal fibres between them contract; almost simultaneously the constriction between a and b relaxes, and the two particles are thus brought swiftly together. A similar process naturally would take place for each of the shifting segments. Thus the function of the longitudinal muscles would be to contract between new rings of constriction and thereby aid in relaxing the former ring between them. During my one observation of the segmenting process, as seen on the surface of the intestine, I could not be sure that the distance between neighbouring segments was shortened as the constriction relaxed; that activity of the longitudinal fibres is present, however, is indicated by observations of Raiser on the intestines of the rabbit and the cat. Raiser observed the outer surface of the coils, and describes the normal movement as an alternate contraction and relaxation of single divisions of the longitudinal fibres; he notes that these short divisions shift. But whether they shift in alternation with the shifting circular constrictions, as seems probable, is an interesting point not yet determined.