DIGESTION 339 



organic substances in small samples of saliva collected from 

 a fresh gland is more nearly proportional to the rate of 

 secretion than is the qjantity of water and salts, which 

 varies also with the blood-supply. 



In order to explain the difference between the cerebral and 

 sympathetic secretion, Heidenhain has supposed the existence of 

 two kinds of secretory fibres: (i) secretory fibres proper, the excita- 

 tion of which causes an actual outpouring of liquid from the gland- 

 cells into the ducts; (2) 'trophic' fibres, which not only promote 

 the changes by which already formed organic constituents of the 

 secretion pass into solution, but also stimulate the growth of the 

 glandular protoplasm. In such animals as the dog the cranial nerve 

 (the chorda in the case of the submaxillary and sublingual glands) 

 was supposed to contain many fibres of group (i), comparatively few 

 of group (2) ; and the sympathetic few of (i) and more of (2). Since 

 these trophic fibres, according to Heidenhain's original statement of 

 his hypothesis, possess two distinct functions, his second group is 

 sometimes subdivided into a set of katabolic fibres which favour the 

 breaking down of material in the cell as a preliminary to its removal 

 in the secretion, and a set of anabolic fibres which have to do with 

 the building up of fresh substance. But it must be remembered that, 

 although it may be convenient for certain purposes to make such a 

 physiological classification, there is no proof of the existence of any 

 corresponding anatomical distinction ; and Langley has shown that 

 in the cat's chorda atropia acts simultaneously on all the secretory 

 fibres ; the moment it paralyzes one group all are paralyzed. If they 

 were anatomically distinct, it might have been supposed that atropia 

 in a certain dose would pick out one or other group, and leave the 

 rest still active. 



It is conceivable that the differences between chorda and 

 sympathetic saliva are due, not to the nerve-fibres, but to 

 the end organs with which they are connected ; 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. However suggestive these facts may be, they will 

 not as yet bear the weight even of a hypothesis of salivary 

 secretion. There must in any case be some overlapping in 

 the nerve-supply ; that is, some cells must be supplied by 



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