EVOLUTION AS SEEN IN THE STRUCTURE OF MODERN ANIMALS 



41 



caecum 



small 

 ntestine 



arqe 

 intestine 



coiecum 



arge intestine 



small intestine 



coiecum 



appendix 



RABBIT 



vermiform appendix 

 in embryo jn adult 



MAN 



FIG. 3.12. Caecum and vermiform appendix in rabbit, in human embryo, and in adult 

 man. (Rabbit, after Bensley, Praciical Anaiomy of fhe Rabbif, 7th ed.. The Blakiston 

 Company, 1945. Man, after Walter, Biology of the Verfebrafes, The Macmillan Com- 

 pany, 1939.) 



with no appendix at all. On the other hand, if we study herbivorous 

 (plant-eating) mammals having simple stomachs more or less like ours we 

 find that the caecum is a large pouch, in some cases as capacious as all 

 the rest of the digestive system put together. In some herbivorous mam- 

 mals it is broad throughout its length. In others it tapers to a point at its 

 free end. The combined length of caecum and appendix in a rabbit, for 

 example, is about 18 inches (Fig. 3.12). For the first 12 inches or so it 

 is a broad, thin-walled pouch containing a spiral fold or valve which in- 

 creases the internal surface. The terminal 5 or 6 inches of it has thicker 

 walls and no spiral valve and corresponds to our appendix. 



The large caeca of herbivorous mammals form storage compartments in 

 which partly digested food remains while bacterial action takes place upon 

 it. One of the most abundant constituents of plant tissue is cellulose. The 

 digestive fluids of mammals contain no enzymes that digest this substance. 

 For this reason man, for example, could derive no appreciable nourish- 

 ment from a diet of paper, a product consisting largely of cellulose. Cer- 

 tain bacteria, however, can break down cellulose into chemical compounds 

 which the body can utilize. In the caeca of herbivorous mammals such 

 bacteria have time to act on the cellulose, thereby retrieving for the ani- 

 mal a portion of its diet which would otherwise be wasted. Accordingly 

 the caecum is a valuable organ for herbivorous animals lacking the com- 

 plicated stomachs of ruminants (e.g., cattle). 



How do we happen to have a caecum and associated appendix? Our 



