272 



DIGESTION 



same amount of N in the food, it is about twice as great for bread as for milk 

 or meat, and in the former case rises to about one-fifth the amount of N ingested. 

 In a carnivorous animal like the dog, the digestion of bread seems to call for a 

 much greater effort on the part of the pancreas than the digestion of meat or 

 of milk (Walther). 



B. MORPHOLOGICAL CHANGES IN THE PANCREAS 



Kiihne and Lea observed directly on the living rabbit the changes which 

 take place in the cells of the pancreas during secretion (Fig. 109). When 

 secretion begins a change of form comes over the cells, which is expressed in 

 a striking change of configuration of the tubule. While in the inactive state 

 the latter appears perfectly smooth on the outer edge, during activity convex 

 swellings which correspond to the separate cells become visible. The resting 

 cells form an optically continuous picture within the tubule, but the active 



FIG. 109. The pancreas of the rabbit, as observed in the living animal, after Kiihne and Lea. 



A, resting state; B, secretion. 



cells are marked off from one another by sharp and, for the most part, double 

 boundary lines. Likewise during the active condition the striations in the 

 outer zone, running from the base toward the inner border, stand out more 

 clearly. The granules of the inner zone withdraw gradually from the region 

 of the nucleus toward the lumen, become smaller and softer, and finally 

 disappear altogether. 



5. THE LIVER AND THE SECRETION OF BILE 

 A. GENERAL PHENOMENA OF HEPATIC SECRETION 



The secretion of bile differs from the other secretory processes thus far 

 studied, primarily by being continuous, but also by the fact that, notwithstand- 

 ing many researches, secretory nerves have not yet been found for the liver. 



In this respect there is complete agreement between the bile and the urine, 

 and one might suppose that these two secretory processes do not require the 

 cooeration of secretory nerves, since in order to discharge their function of 

 removing excretory products from the body they must go on continuously. As 



