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Show 20 DR. E. LONNBERG ON DIGESTIVE [Jan. 14, transversely in the colon as the latter is apt to do, but longitudinally, so that one plica extends down the colon, the other m an opposite direction on the other side. I am, however, inclined to think that these plicae morphologically correspond to the more or less developed caeco-colic valve of other related forms. Although the direction is different the connection with the ileo-caecal valve may decide in favour of such an interpretation. These plicae do not form any boundary between the caecum and the colon. They fi inn no boundary at all directed as they are now. But how could they get such a direction ? I think this may be explained in connection with the shape of the caecum. Owen, in describing the intestine of the Wombat, used the following words : - " The caecum is extremely short but wide; it is remarkable for being provided with a vermiform appendage."' Later authors have adopted this same interpretation, but I hardly think it is right. If it had been a true vermiform appendage, that is, the reduced blind end of a caecum, it ought to have opened into the caecum of which it itself was a part. But it does not, as has been already stated above. It opens with a quite independent opening of its own near that of the ileum. I judge from this that the so-called 2~>rocessus vermiformis of the W o m b a t represents a rudiment of the whole ccecum. If we assume that a moderately developed caecum should for some reason or another become reduced to a mere appendage, it must acquire a similar situation and open into the colon close to the ileum. It might then easily happen that the wall of the terminal portion of the rudimentary caecum became fused with the wall of the ileo-caecal valve. Such an event might be the more easily effected as the shortened mesentery of the caecal rudiment would draw the latter more and more to the ileum and make both more closely connected. It would also be more convenient if the two openings into the colon lay near each other and were parallel in direction, because there would then be less risk of particles of food entering the caecal rudiment. I believe, indeed, that such a retrograde development has really taken place, and that in the ancestors of the W o m b at the caecum has been reduced to a rudiment, which might happen if they lived on such a diet that the caecum was not needed for the digestion of the food. W h e n the caecum had already reached a considerable degree of reduction, the diet of the animals was changed, and they began by-ancl-bye to feed on harder and less easily digestible vegetable matter containing cellulose, etc. The caecum was now, however, so rudimentary that it could not as in other related forms which live on a similar diet, help in the digestion of this food-stuff. This function became, therefore the duty of the colon alone, which in consequence had to be considerably enlarged. It grew in strength, and its capacity increased so that it would be able to hold the greatly augmented amount of the less nourishing food that was needed for the sustaining of life and growth. The colon was then distended by the large quantities 1 Owen : ' Anat. of Vertebr.' p. 417. |