12 TISSUES AND ORGANS 



reticulo-endothelium in general, because, although in the liver, 

 they are sensu stricto not of it. 



PANCREAS 



There is nothing new to report as to the microscopic characters 

 of the pancreas proper, but the islets of Langerhans present points 

 of interest, especially in connection with the relatively recent 

 discovery of insulin. 



All the cells of the islets contain granules, which at first sight 

 do not differ from those of the externally secreting cells. Closer 

 investigation shows that there are two types of granules in the 

 islets. A-granules are precipitated by alcohol and lie in cells 

 with large spherical nuclei containing little chromatin. Cells 

 containing these granules lie in the middle of the islets, and may 

 be diffusely filled with granules or almost all these may be aggre- 

 gated on the side of the cell nearest to the capillary. B-granules 

 are precipitated by watery solution of chromic acid and sublimate. 

 They are much smaller than the A-granules, and cells containing 

 them are much more numerous than are the A-cells. The nucleus, 

 which is central, is small and densely chromatic. Lastly there are 

 some cells with no granules ; but there are some with a very small 

 number of A-granules. It is therefore supposed that the A-cells 

 can arise from these non-granular elements. The old view that islet 

 cells could be converted into acinar cells has been disproved by 

 Bensley, who has shown clearly that the former are specifically 

 differentiated elements incapable of differentiation into any other 

 form. 



Even the mitochondria show distinct differences between the 

 acinar and islet cells. In the former they are long relatively 

 coarse filaments, whereas in the latter they occur as very delicate 

 filaments and as very fine granules. 



In spite of the specific characters of the islet cells it must be 

 recalled that the human embryo of about three months shows the 

 islets connected with lumina by solid stalks ; in still-born infants 

 such a connection is sometimes persistent. 



In diabetes various changes have been described in the islets, 

 but even so some severe cases seem to have shown no pathological 



