CHAPTER II. DIGESTION. 



1. The Alimentary Canal is a tube, beginning at the 

 mouth and ending at the anus, along which the food passes, 

 in which it undergoes digestion, and from which all the 

 available portion is absorbed into the blood, leaving the 

 residue to pass on and out by the anus. At first sight, in 

 dissecting, this canal appears hopelessly complicated, but in 

 the embryo (i.e. the rudimentary rabbit before birth) it is a 

 practically straight tube of nearly uniform diameter, running 

 from mouth to anus. In the head and neck, where there is 

 no Cffilom, and in the thorax, where the coelom is occupied 

 by other organs (heart and lungs), the alimentary canal 

 remains straight in the adult ; but in the abdomen, where 

 the coelom is spacious, and has not much else in it, the 

 alimentary canal bends and coils and varies in calibre 

 in such a way that the maximum of digestive area is packed 

 away in the minimum of space. The coiling is, however, 

 on a perfectly regular plan. Just as you cannot tie knots, 

 but only make loops on a string both whose ends are 

 fastened, so, as the abdominal portion of the rabbit's ali- 

 mentary canal increases in length during development, it 

 can only become looped in certain ways. Indeed, it is even 

 more confined than the string, since it is united to the 

 dorsal middle line by the mesentery. The nature of the 

 loops and coils of this part of the canal is diagrammatically 

 shown (a little simplified, and very much out of proper 

 proportions), in fig. 1. 



2. Food-Materials. Before proceeding to a detailed 

 description of the canal, and of the processes to which 

 the food is subjected while in it, we must briefly explain 

 the various classes of substances that are found in the 

 rabbit's food. They may be roughly classed thus, on a 

 chemical basis : 



(1) Proteids and allied substances, highly complex organic 



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