266 SECRETION OF PANCREATIC JUICE. [BOOK n. 



secretion. The quantity secreted in 24 hours by man has been 

 calculated at 300 c.c. Like the salivary glands, the pancreas 

 while secreting is flushed, through dilation of its blood-vessels. 



The secretion if present may be increased, or if absent may be 

 called forth, by stimulation of the medulla oblongata, and when 

 going on may be arrested by stimulation of the central end 

 of the vagus through a reflex act, the efferent channels of 

 which have not yet been made out; probably the arrest of the 

 secretion which is said to be caused by nausea or vomiting is thus 

 brought about by stimulation of the vagus endings. These facts 

 shew that the secretion is under the influence of the central 

 nervous system ; but we have no such satisfactory knowledge of the 

 exact working of the nervous mechanism as in the case of the 

 salivary glands. 



SuccilS entericus. With regard to the secretion furnished by 

 the intestine itself our information is very limited. The secretion 

 of the isolated intestine appears to be not a constant one, but to 

 need for its production some stimulus (mechanical or other) which 

 probably acts in a reflex manner. After section of the nerves 

 going to a piece of intestine isolated after Thiry's method, a 

 copious flow of a dilute intestinal juice is said to take place. 



Thus, while the influence of the nervous system is in the case 

 of the submaxillary gland tolerably clear, in the case of the other 

 secretions we have yet much to learn, and we must rest rather 

 on analogy with the submaxillary gland, than on any known facts. 

 We cannot, however, go far wrong, if we conclude that in all cases 

 secretion is essentially due to a direct activity of the epithelium 

 cells, and that variations in the blood-supply have a secondary 

 effect only. 



We may now pass on to the second problem. What is the 

 exact nature of the activity which is thus called forth ? 



Towards the solution of this problem much progress has been 

 made by the study of the microscopical changes in secreting glands 

 during various stages of activity and rest. And these are perhaps, 

 in some respects, best shewn in the pancreas. 



It is possible, by special precautions, to examine with even 

 high powers of the microscope the pancreas of an animal such 

 as the rabbit, while still alive with the circulation intact; and 

 thus to watch the changes going on both when the animal has 

 been deprived of food for some time and the gland is therefore 

 at rest, and when the animal has been recently fed so that digestion 

 is going on and the pancreas in consequence is engaged in pouring 

 its secretion into the duodenum. In the former case, i.e. when the 

 pancreas is at rest and little or no secretion is being poured out, 

 the following appearances may be recognised. The outlines of 

 the individual cells forming an alveolus (Fig. 50 A) are very in- 

 distinct, and each cell is loaded with a number of small highly 



