178 ANIMAL BIOLOGY. [Part I. 



within the body, are still in a sense outside it. They are in the 

 same position as the fuel in a Cornish or Lancashire boiler 

 which is placed in the tube or tubes that run through the midst 

 of the boiler, but which can hardly be said to be inside it. 

 Hence the elaborated food-stuff has to be absorbed through the 

 walls of the canal before it enters the body. 



But though there are no tubes which carry material from the 

 canal into the system, there are several large, and a multitude 

 of minute tubes which convey material into the canal. These 

 are the ducts of the various glands which minister to digestion. 

 They are divided into : (1) salivary glands in the region of the 

 mouth; (2) gastric glands in the stomach; and (3) intestinal glands. 



The digestive tube itself has the following structure (for 

 details see Chapter V. p. 78, Fig. 28) : Externally there is a 

 peritoneal investment; then follow the muscular walls with 

 longitudinal and circular fibres; within this is the mucous 

 membrane. The epithelium that lines the tube passes up into 

 and lines the ducts of the glands, large and small. At the 

 mouth and vent it is continuous with the epidermis. 



This epithelium, and its supporting mucosa and sub-mucosa, 

 does not form a smooth and even lining to the canal. In the 

 stomach of frog and rabbit it is thrown into folds or rugce. In 

 the small intestine of the rabbit it is doubled inwards so as to 

 form crescentic or nearly circular folds (valvulw conniventes). 

 And if a small portion of the small intestine of a rabbit or fowl 

 be examined under water with a lens it will be seen to have a 

 velvety appearance, due to great numbers of closely-set minute 

 processes, the villi (see Fig. 28). In the small intestine of the 

 'frog there are minute dependent processes ; but they do not 

 form so close-set a velvety pile as the villi of fowl or rabbit. 

 In the large caecum of the rabbit there is a spiral infolding 

 of the tube readily seen when this curious appendage is re- 

 moved, cleaned out by directing a stream of water through it, 

 distended with air, and dried. In the colon of the rabbit 

 the whole wall is characteristically puckered. 



In the fowl, answering to the stomach of frog or rabbit, there 

 is anteriorly a proventriculus, the walls of which are richly sup- 



