CHAP. L] TISSUES AND MECHANISMS OF DIGESTION. 385 



tubular and often simple, which have a very narrow lumen, and 

 which possess central and ovoid cells, while the pyloric end contains 

 glands which are branched, which have a relatively deep mouth 

 and wide lumen, and which possess one kind of cells only, central 

 cells or cells very like these. In the middle region of the stomach 

 the one kind of gland gradually merges into the other ; in passing 

 from the cardia to the pylorus the ovoid cells become less numerous 

 and at last disappear, the mouth becomes longer, the lumen wider, 

 and the body of the gland becomes more and more branched. 



The above supplies a general description of the gastric glands 

 but these vary in minor characters and to a certain extent in 

 distribution in different animals ; and as we shall presently see 

 in all cases, the glands vary in condition and so in appearances 

 according as digestion is or has been going on in the stomach. 



The Salivary Glands. 



215. The structural differences between the ' mucous ' cells 

 lining the mouth and the 'central' and 'ovoid' cells lining the body 

 of a gastric gland lead us to infer that the former differ from the 

 latter in function ; and we have other evidence that this is so, that 

 it is the central and ovoid cells which actually secrete the gastric 

 juice, and that as far as the gastric juice is concerned, the mouths 

 of the glands serve chiefly (though the mucous cells have a purpose 

 of their own) to conduct to the interior of the stomach the juice 

 secreted by the body of the gland. We may therefore speak of 

 the body as the secreting portion and the mouth as the ' duct ' of 

 the gland. 



This distinction between a secreting portion and a conducting 

 portion, more or less obvious as we have said in most glands, is 

 especially striking in the case of the salivary glands. These are 

 involutions of the (epiblastic) mucous membrane of the mouth as 

 the gastric glands are involutions of the (hypoblastic) mucous 

 membrane of the stomach ; but instead of being comparatively 

 simple they are exceedingly branched racemose glands, and the 

 secreting portion of the gland is removed to a great distance from 

 the epithelium of the mouth so that the conducting portion is 

 of very great length. Moreover, not only the epithelium lining the 

 secreting portion but also that lining the conducting portion differs 

 so completely from the epiblastic epithelium lining the mouth 

 that we may study the structure of the gland quite apart from the 

 structure of the lining of the mouth, whose sensory functions, in 

 the way of taste for instance, are so much more important than its 

 digestive functions that we may reserve the study of its features 

 until we come to deal with the senses. 



A salivary gland such as the submaxillary consists of a long 

 main duct which pursues an undivided course backwards for 

 several centimetres from its opening into the cavity of the mouth 



F. 25 



