II 



EXTERNAL DIGESTIVE SECRETIONS 



85 



cent) a third (Hess). In the rabbit there is a secondary duct besides 

 the duct of Wirsung, but it is of no importance in the adult ; 

 U. Lombroso (1907) showed that ligation of the principal duct 

 alone causes diffuse alterations in all the organs, co-extensive with 

 those observed when the secondary duct is also occluded. 



When freshly examined, the secreting cells at the end of the 

 alveoli show a clear, homogeneous, outer zone, covered with the 

 basement membrane, and a granular, somewhat clouded, inner 

 zone, which is turned towards the lumen. In very fresh prepara- 

 tions the granules extend over the whole of the cells, but if 

 the preparation is cooled they are packed towards the lumen. 

 In quite fresh preparations, again, the nucleus is invisible, or 

 scarcely seen. In carmine 

 preparations only the outer 

 zone and the nucleus stain 

 (Fig. 26). 



In the middle of the al- 

 veolus Langerhans (1869) 

 discovered spindle-shaped 

 cells with a homogeneous 

 body, sharp outline, and 

 a large clear nucleus, 

 known from their position 

 as the centro-acinar cells. 

 Saviotti, Boll, Ebner, v. 

 Frey, Giannelli and many 

 others, showed by system- 

 atic investigation of various kinds of animals that these cells 

 are constant in all vertebrates. Such centro-acinar cells are more 

 numerous at the neck of the alveolus than at its base, where they 

 may be entirely absent. 



The nature of these cells has been the subject of much dis- 

 cussion. Langerhans, Saviotti, Heidenhain, regard them as 

 epithelial cells. Pflliger takes them to be nerve cells. Many 

 authors have supposed them to be connective tissue ; but all these 

 surmises have now been abandoned, and after the histogenetic 

 studies of Laguesse and his pupils, their epithelial character is 

 generally admitted. 



According to Renaut and Laguesse the centro-acinar cells are 

 the essential factors in all the changes of form that occur within 

 the gland. They also participate in the external secretion. 



In addition to the epithelial cells which constitute the alveolar 

 gland proper (acinar cells, centro-acinar, epithelial cells of ducts) 

 the pancreas of all vertebrates presents areas of tissue or com- 

 pact structures, which are distinct in character from the alveoli. 

 They stain much less freely with ordinary methods; in some 

 cases the individual cells have no sharp outline but resemble a 



G 2 



FIG. 26. 



