THE FUNCTIONS OF THE DIGESTIVE ORGANS. 489 



glucosamine, while in the protein bodies present in serum there is but a 

 small amount of this aminohexose. It is conceivable that the albuminous 

 substances in blood-serum contain perhaps unknown preliminary stages 

 in the glucosamine formation, but it is, however, also possible that we have 

 here one stage in the process of the conversion of an amino acid into a 

 sugar. At all events, the formation of mucin with its peculiar composition 

 deserves considerable attention. 



It is not to be assumed that during the activity of the gland,' the cells 

 must be entirely built up as well as the secretion formed. At the same 

 time we should not think for a moment that these cells are permanent 

 structures. We do not doubt that they are constantly being renewed like 

 all other cells of the body, and that here and there a cell disappears to 

 be replaced by a new one. 



The decrease in the volume of the individual cells during their activity 

 also has an effect upon the weight of the gland. If the submaxillary 

 gland is brought into activity by stimulating the fibers of the facial nerve, 

 it is found that the active gland decreases in weight. 



Some attention is due to the question as to the manner in which inner- 

 vation of the salivary glands is normally produced. The Russian physi- 

 ologist Pawlow deserves great credit not only for having developed the 

 operative technique so that it is possible under purely physiological con- 

 ditions to trace the functions of the various digestive glands, but also for 

 having shown in an entirely original way how the activity of the same is 

 dependent upon definite external conditions. We have known for a long 

 time that the activity of the salivary glands is influenced by sensations 

 of taste and smell, and even by certain imaginations. Pawlow, however, 

 deserves the credit for clearly demonstrating by experiment the remark- 

 able ability that the salivary glands have for adapting themselves to their 

 work. Among other things he called attention to the following observa- 

 tions: If a dog is fed with dry, solid nourishment, there at once takes place 

 a considerable flow of saliva; while on the other hand, if the nourishment 

 is liquid, there results but a slight flow. Chemicals which act as irritants, 

 such as acids and alkalies, increase the flow of saliva in proportion to the 

 irritation produced. The organism attempts in this way to protect itself 

 from the action of such substances. It dilutes them and washes them out 

 from the mouth as much as possible. If small quartz pebbles are placed 

 in the mouth of a dog, the animal permits them to drop out gradually 

 from his mouth, but without any flow of saliva. If, on the other hand, the 

 same material is placed in the dog's mouth in the form of a powder, there 

 is a considerable flow of saliva. The purpose of this is clear, in this way 

 the sand is washed out of the mouth, while in the case of the little stones 

 the tongue alone can accomplish their removal. In all such cases we are 

 struck with the utility of the whole mechanism. This is still more marked 



