DIGESTION IN THE MOUTH 715 



blood vessels could be regarded as endowed with the properties of a semi- 

 permeable membrane, we should need, in order to effect the separation, a 

 pressure ten to twenty times as large as the arterial blood pressure. We 

 must conclude then that work, both osmotic and mechanical, is performed 

 in the separation of the fluid from the blood and its transference in the 

 form of saliva to the duct. A very simple experiment will suffice to show 

 that this work must be effected, not by the endothelial cells of the blood- 

 vessels, but by the gland cells themselves. The fluid passes from the 

 blood vessels first into the lymphatic spaces, whence it is taken up by 

 the secretory cells. If the first act in secretion consisted in an increased 

 exudation of fluid from the blood into the lymph spaces, the first effect of 

 exciting the chorda tympani nerve should be to cause an accumulation of 

 fluid in the lymph spaces, some increase in the lymph flow from the gland, 

 and the swelling of the whole gland. By placing the gland in a plethysmo- 

 graph we can record the actual changes in its volume which ensue on 

 excitation of the chorda tympani. If all secretion be prevented by the 

 previous administration of atropine, stimulation of this nerve produces, as 

 might be anticipated, an increased volume of the gland in consequence of the 

 dilatation of its vessels (Fig. 338). If however the gland be allowed to 

 secrete we obtain, in spite of the simultaneous increase in size of the vessels, 

 an actual diminution in the size of the gland itself, showing that the first 

 effect of the stimulation is on the cells of the alveoli (Fig. 339). Under the 

 stimulus these empty themselves of the fluid they contain, replenishing their 

 loss at the expense of the fluid in the lymph spaces. The increased passage 

 of fluid from blood to lymph space is therefore a secondary and not a primary 

 effect of the nerve stimulation, and the first effect on the gland is a 

 diminution of volume and not an increase, as one would expect if the vascular 

 endothelium were primarily responsible for the act of secretion. 



HISTOLOGICAL CHANGES DURING SECRETION 



The process of secretion is associated with marked changes in the structure 

 of the cells- composing the secretory alveoli. The changes are of the same 

 general character whatever class of glands we investigate, though the ease 

 with which they are to be demonstrated varies with the reactions of the 

 various glands to the hardening fluids usually employed. If a small frag- 

 ment of a mucous gland be teased in blood serum or in 2 per cent. NaCl 

 solution, the cells are found to be packed with a mass of coarse, highly refrac- 

 tive granules. (Fig. 340). If a corresponding specimen be made from a 

 serous gland (Fig. 341) the cells are also packed with granules, which, how- 

 ever, are much finer in structure. On making similar specimens from 

 glands which have been forced to secrete for six or seven hours, the individual 

 cells are found to be much smaller and the protoplasm of the cell is absolutely 

 or relatively increased in amount, while the granules are much fewer and are 

 now confined almost entirely to the inner margin of the cell. Activity is 

 thus associated certainly with a discharge of granules, and probably with 

 some increased building up of protoplasm. We may regard the act of 



