Fig. 47. A transsection of the wall of the anterior region of the small intestine of the hibernating animal, under low magnification; ln, lymph node; tp, tunica propria; other letters as in Figure 42.
The circular, cm, and longitudinal, lm, muscle layers are compact, and are distinct from the other layers of the wall; the former is approximately twice the thickness of the latter. The relative thickness of all the layers in the three regions of the intestine may be seen by comparing Figures 47, 48, and 49.
Fig. 48. An outline of a transsection of the wall of the middle region of the small intestine of the hibernating animal, under low magnification; lettering as in Figure 42.
The serosa, s, which is of about the same character in the three regions under discussion, is a distinct and fairly dense layer of connective tissue with numerous blood-vessels.
The general appearance of the mucous membrane as a whole is sufficiently clear in the low-power drawing described above, so that all that need be shown under a higher magnification is the epithelium (Fig. 50). The upper part of this figure represents the lower end of one of the intestinal glands cut longitudinally, below which is the end of another gland in transverse section. Between the two sections is the compact tunica propria of lymphatic tissue.
The section from which this particular figure was drawn was in the anterior region, but the corresponding part of a section in either of the other regions would have practically the same appearance.
The epithelium is of the stratified columnar type. The superficial cells are very tall and narrow, with the nuclei generally at or near the bases, though an occasional nucleus may be seen near the free end of a cell. Below the tall columnar cells are four or five rows of nuclei which represent smaller, irregular cells, though the cell walls could not always be determined between the closely packed nuclei. No goblet cells are to be seen at any place.
Fig. 49. An outline of a transsection through the wall of the posterior region of the small intestine of the hibernating animal, under low magnification; lettering as in Figure 42.
The relative diameters of the three regions of the small intestine in the feeding condition are about the same as noted for the hibernating stage; that is, the anterior region has the greatest diameter and the other regions are smaller and have about the same average diameter.
The most marked difference between the intestine during hibernation and feeding is in the relative thickness of the mucosa and muscular layers. As described for the hibernating stage, so in the feeding stage, the mucosa is relatively the thickest in the anterior regions and diminishes in thickness caudad; but while, in the hibernating stage, it forms, in the anterior region, as much as half of the entire thickness of the wall, in the feeding condition it forms, in the same region, at least two thirds of the entire wall and in the middle and posterior regions more than half of the wall.
Fig. 50. Part of the mucous membrane of the anterior region of the small intestine of the hibernating animal, under high magnification. The upper part of the figure shows a part of a gland cut longitudinally, the lower part of the figure shows another gland cut transversely; e, epithelium; tp, tunica propria.
The feeding animals being the smaller, the diameter of the intestine was considerably less than in the hibernating stage; but the actual thickness of the mucosa was practically the same, so that the difference in diameter was due to the difference in the thickness of the muscular and fibrous layers. It is therefore probable that the differences noted above are due rather to the differences in the size of the animals from which the tissues were taken than to the different conditions of hibernation and feeding. The point to be noticed is that the increase in the diameter of the intestine is due almost if not entirely to an increase in thickness of the connective tissue and muscle layers.
No difference in the complexity of the folds of the mucosa of the two stages can be noticed.
The thickness of the fibro-muscular part of the wall of the intestine varies considerably on different sides of the same region, but it consists of the same layers in about the same relative amounts.
Fig. 51. An outline of a transsection of the wall of the middle region of the small intestine of the feeding animal, under low magnification; m, mucosa; other letters as in Figure 42.
Figure 51 represents in outline the wall of the middle region of the small intestine during feeding.
The epithelium is of the same thickness in the two stages, and the only difference in its character that can be seen under a high magnification is that, in the middle region at least, the nuclei are not crowded so close together at the basal ends of the cells as in the hibernating stage but are scattered more towards their free ends.
Fig. 52. A transsection of the wall of the anterior region of the rectum or large intestine of the hibernating animal, under low magnification; tp, tunica propria; other letters as in Figure 42.
Altogether, the differences in microscopic structure between the small intestine of an alligator at the end of the hibernating period and at the end of a period of regular feeding are very slight.
The Large Intestine.
The planes of the two sections studied are shown in Figure 35; a low-power drawing of the posterior region is shown in Figure 52. The anterior and posterior regions of the large intestine do not differ from each other sufficiently to make it worth while to represent both by drawings. Had an entire section through either region been drawn it would be seen that the wall is of very different thickness in different places, as was noted in connection with the small intestine; the posterior section was drawn where the wall was thin.
It might be supposed that in the feeding season the fecal matter in the posterior region of the rectum would stretch the walls sufficiently to obliterate largely the prominent folds seen in Figure 52, but such does not seem to be the case. The usual layers of the vertebrate intestine are present.
Fig. 53. The epithelium of the anterior region of the rectum of the hibernating animal, under high magnification; e, epithelium; tp, tunica propria.
The epithelium, shown under high magnification in Figure 53, is of the same character and thickness throughout, except that as the anal aperture is approached the columnar epithelium changes into the stratified variety. It consists of very tall and narrow columnar cells apparently in one layer, though it is difficult to be sure of this. With an occasional exception, near the top, all of the nuclei are arranged in a fairly wide zone below the middle of the epithelium. The nuclei are oval in shape and lie so close together that it is difficult, as has been said, to be sure that the cell to which each belongs extends throughout the entire thickness of the epithelium.
Beneath the epithelium (Fig. 52, e) is a dense tunica propria, tp, underlaid, in turn, by the muscularis mucosa, mm, and a submucosa, sm, of the usual character, which is thrown into marked folds. The circular, cm, and longitudinal, lm, layers are of the usual character except that they vary more in thickness, as noted above, and in density than is usually the case.
The serosa, s, is comparatively thin and compact in both regions, and varies somewhat in thickness at different places.
The large intestine of the feeding animal was sectioned in the same regions as in the hibernating. As has been said, the feeding animals used were much smaller than the hibernating, so that, as might be expected, the diameter of the large intestine was much less in the former than in the latter. Except for this difference in diameter there was no noticeable difference between the two stages. In the case of the small intestine, it will be remembered, the greater diameter of the intestine of the larger animal was mainly due to the greater thickness of the muscular and connective-tissue layers and not to any increase in thickness of the mucous membrane. In the large intestine the mucosa varies in thickness in the animals of different size as do the other layers of the wall.
The glandular character of the lining of the large intestine seems to indicate that this region of the intestine must have some digestive or absorptive function and that it does not act merely as a receptacle for fecal matter; this makes it all the more strange that there should not be some change produced in its structure by five or six months of feeding or of fasting.
Summary.
The material used in this investigation was taken from young animals at the end of a feeding period of about five months, and towards the end of the hibernating period after fasting for four or five months.
The regions of the enteron that were studied were as follows: the tip and base of the tongue; the anterior and posterior regions of the roof of the mouth; the anterior and posterior regions of the œsophagus; the cardiac, fundic, and pyloric regions of the stomach; the anterior, middle, and posterior regions of the small intestine; the anterior and posterior regions of the large intestine. Since the work was started at the end of the hibernating period, the tissues of that period were studied and drawn first.
The only difference between the structure of the tip of the tongue during hibernation and during the feeding season is that the scaly epithelium with which it is covered is somewhat thicker and more compact in the former than in the latter condition, though even this difference may have been due to differences in the ages of the animals used. The base of the tongue differs from the tip in having a thicker epithelium and in having compound tubulo-alveolar glands. These glands in the hibernating animal have many more alveoli than in the feeding animal, though this, again, may have been due to the difference in age.
The lining of the roof of the mouth is essentially the same as that of the tongue. The glands are found only in the posterior region. The slight differences in the papillæ here found may easily be due to the difference in age.
The œsophagus shows the usual layers for that region. Its epithelium is partly ciliated in the anterior part. The muscularis mucosa is very scant in the anterior region. The only difference between the two stages is that in the feeding the muscularis mucosa in the anterior region is much more strongly developed than in the hibernating stage; and in the former the nuclei of the epithelium are not arranged in two zones as in the latter.
The stomach has the usual layers, and has essentially the same structure in the three regions studied, except that the wall in the fundic region is much the thickest, due mainly to the great thickness of the middle muscle layer. Only one kind of cell is found in the gastric glands. No difference is to be noted between the hibernating and feeding conditions.
The chief peculiarity of the small intestine is the apparent entire absence of the submucosa. Goblet cells are also wanting. The greater diameter of the anterior region is due both to the greater diameter of the lumen and to the greater thickness of the walls. The middle and posterior regions have about the same diameter, though the mucosa becomes thinner and less complicated caudad. There is practically no difference between the hibernating and feeding stages.
The anterior and posterior regions of the large intestine have essentially the same structure. No difference can be seen between the hibernating and feeding conditions.