368 A MANUAL OF PHYSIOLOGY 



saliva collected from a fresh gland is more nearly proportional 

 to the rate of secretion than is the quantity of water and salts, 

 which varies also with the blood-supply. 



Lest the apparently insignificant result of artificial stimulation 

 of the sympathetic in such animals as the dog should cause its 

 secretory action to be appraised at too low a value, it should 

 be remembered that in the intact body the sympathetic secretory 

 fibres, when they are excited, are, it may be assumed, excited 

 independently of the vaso-constrictors, and even in conjunction 

 with the vaso-dilators of the salivary glands. 



It is conceivable that such differences between chorda and 

 sympathetic saliva as are not accounted for by the differences in 

 the blood-flow during their stimulation are due, not to the 

 nerve-fibres, but to the end organs with which they are con- 

 nected ; that is, the two nerves may supply, not the same, but 

 different gland-cells. And it is well known that even after 

 prolonged stimulation of the chorda or chordo-lingual alone, 

 some alveoli of the dog's submaxillary gland remain in the 

 ' resting ' state ; after stimulation of the sympathetic alone, the 

 number of unaffected alveoli is much greater ; while after 

 stimulation of both nerves, few alveoli seem to have escaped 

 change. If there is no essential difference between the cranial 

 and sympathetic secretory fibres, it is easy to understand that 

 they will be distributed to different secreting elements. The 

 supposed proof that there must be some overlapping in the 

 nerve-supply i.e., that some cells must be supplied from both 

 sources, since excitation of the sympathetic influences the 

 amount of organic material in the saliva obtained by subsequent 

 stimulation of the chorda is, as we have just seen, by no means 

 so cogent as has been assumed. And, indeed, we know nothing 

 of a division of labour between the cells of a gland, except when 

 there are obvious anatomical distinctions. Thus, the sub- 

 maxillary gland in man contains both serous and mucous acini, 

 and mucin-making cells are scattered over the ducts of most 

 glands, and, indeed, on nearly every surface which is clad with 

 columnar epithelium. In these cases we cannot doubt that one 

 constituent mucin of the entire secretion is manufactured by 

 a portion only of the cells. In the cardiac glands of the stomach, 

 too, the ovoid cells, in all probability, yield the whole of the 

 acid of the gastric juice. But, so far as we know, every hepatic 

 cell is a liver in little. Every cell secretes fully-formed bile ; 

 every cell stores up, or may store up, glycogen. So it is with 

 the secretory alveoli of the pancreas, if we consider the islands 

 of Langerhans as having no connection with the alveoli ; one 

 cell is just like another ; all apparently perform the same work ; 

 each is- a unicellular pancreas. (But see p. 554). 



