CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 391 



the food is for a distance exposed to the action of the former before 

 it meets with the latter. 



The structure of the ducts is, in all essential points, similar to 

 that of the ducts of a salivary gland, save that the striation of the 

 epithelial cells is less distinct. ^cs in the case of the salivary 

 gland, the ductule, or narrow terminal portion of the duct, just as 

 it joins the alveoli is lined by flat spindle-shaped cells. 



The alveoli also are similar to those of a salivary gland save 

 perhaps that they are relatively longer and more tubular ; the 

 lumen in all cases is very narrow. As compared with a salivary 

 gland the alveoli are relatively more numerous than the ducts, so 

 that in a section of the gland 1 relatively fewer ducts are seen cut 

 across. Each alveolus is lined with one kind of cell only, which is 

 much more similar to an albuminous than to a mucous cell ; fthere 

 are no demilune cells. The more minute features of the alveolus 

 differ according as the gland has been ' resting ' and so is ' loaded,' 

 or has been ' active ' and so is ' discharged.' The cells lining the 

 alveolus are more or less polyhedral in form, and each cell consists 

 of a clear transparent cell-body, in which occur a number of 

 refractive discrete "granules;" a spherical nucleus lies at about 

 the outer third of the cell. In a ' loaded ' cell these granules are 

 very abundant, and reach from the narrow, inconspicuous lumen to 

 near the outer margin of the cell, so as to leave only a narrow 

 clear transparent zone immediately bordering on the basement 

 membrane ; the cell-substance is so thickly studded with these 

 ' granules ' that the nucleus is completely hidden, and the greater 

 part of the cell appears quite dark. In a ' discharged ' cell these 

 granules are far less numerous, and are largely confined to the 

 inner part of the cell abutting on the lumen, so that there is 

 established a clear distinction between a narrow inner " granular " 

 zone and a clear transparent outer zone, free or nearly free from 

 granules. The width of the granular zone varies in fact with the 

 condition of the gland ; when the gland has been very active the 

 granular zone is very narrow, when moderately active, it is broader, 

 and when the gland has been for some time wholly at rest and is 

 therefore loaded, the granular zone may encroach on nearly the 

 whole cell. But we shall have to return to these matters 

 presently. 



In the pancreas of the rabbit and some other animals groups 

 of cells of a peculiar nature may be seen intercalated at intervals 

 in the midst of the true glandular substance. These are rounded 

 or polyhedral in form, and have a clear cell-substance with a 

 relatively large nucleus ; they do not form alveoli and they have 

 no ducts. Each of these groups is supplied with blood vessels 

 forming a capillary network more close set than elsewhere. The 

 exact nature of these cells is at present a matter of doubt. 



The pancreas is supplied with nerves coming from the solar 

 plexus, and consisting partly of medullated and partly of non- 



