186 



THE ESSENTIALS OP HISTOLOGY. 



network of convoluted capillary vessels ; their function is unknown, but 

 their presence is very characteristic of the pancreas. 



The cells which line the alveoli are columnar or polyhedral in 

 shape. When examined in the fresh condition, or in osmic prepara- 

 tions, their protoplasm is filled in the inner two-thirds with small 

 granules, but the outer third is left clear (fig. 219, A). After a period 



FIG. 219. PART OF AN ALVEOLUS OF THE RABBIT'S PANCREAS. A, AT REST ; 

 , AFTER ACTIVE SECRETION. (From Foster, after Kiihne and Lea.) 



a, the inner granular zone, which in A is larger and more closely studded with fine granules 

 than in B, in which the granules are fewer and coarser ; 6, the outer transparent zone, 

 small in A, larger in B, and in the latter marked with faint strise ; c, the lumen, very 

 obvious in B, but indistinct in A ; d, an indentation at the junction of two cells, only 

 seen in B. 



of activity the clear part of the cell becomes larger, arid the granular 

 part smaller (B\ In stained sections the outer part is coloured more 

 deeply than the inner. 



In the centre of each acinus there may generally be seen some 

 spindle-shaped cells, the nature of which (whether epithelial or connec- 

 tive tissue) has not been determined (centro-acinar cells of Langerhans). 



