SECRETION 345 



a chemical substance produced in the stomach itself by certain constituents of the 

 food, and that this substance acts through the intermediation of the blood current 

 (see page 372 below), although the gastric glands are also powerfully excited by 

 fibres in the vagus nerve. 



Secretion of Bile is produced by the same acid extract of duodenum which 

 excites the pancreas, but whether the same " secretin " is at work we cannot state. 

 The liver can also be excited to secretion by injection into the blood of bile- 

 salts ; in such a case, the concentration in the blood of the constituents of the 

 secretion plays a part in determining the activity of the cells in transferring them 

 from the blood to the duct. The blood supply of the liver (Heidenhain, 1880, 

 pp. 259-268), both in respect of rate of flow and of pressure, affects the rate of 

 secretion to a large extent. 



The Succus Entericus is secreted in a particular section of the small intestine 

 when trypsin is present in a part preceding it in the normal direction of the 

 passage of food. The work of Pavlov (1901, p. 161 of the English edition) tends 

 to show that this is a chemical mechanism. 



The Nervous Mechanism of Secretion. The majority of glands, including those 

 with internal secretion, are supplied with nerves by which they can be excited 

 to action by reflexes from the central nervous system. Most of our knowledge 

 is derived from the study of the salivary glands, owing to the comparative ease 

 with which experimental work can be conducted on these organs. We will 

 consider, in the first place, the special case of the submaxillary gland of the dog 

 and afterwards apply the results to other glands. This submaxillary gland is supplied 

 by two sets of nerve fibres, both of which play a part in the secretory mechanism. 

 The first set, contained in the chorda tympani nerve, arise from the brain in the 

 small-fibred portion of the facial nerve, corresponding to the intermediate nerve 

 of Wrisberg in man, which leaves the mid-brain between the facial and auditory 

 nerves (see Gaskell, 1889, p. 172). 



These fibres may be called the cerebral supply ; the other set comes from the 

 sympathetic system, a special outflow of nerves to the viscera, blood vessels, and 

 similar structures ; to this system of nerves attention will be directed in a 

 latec chapter. 



It is found that excitation of the chorda tympani nerve produces a copious 

 watery secretion, while that of the sympathetic nerve produces a small quantity 

 of a very thick saliva. Heidenhain (1868, p. 113) propounded the view that 

 there are two different kinds of fibres concerned, one set with the secretion of 

 water, together with the diffusible salts present in the blood, and the other set 

 with the formation of the specific solid constituents of the secretion. On this 

 ground he called the former " secretory," the latter " trophic," using this latter 

 word in a rather special sense (1868, pp. 101-104, and 1880, p. 51). The 

 two kinds of fibres are supposed to be present in both nerves, but in different 

 relative amount, which varies according to the kind of animal. In the cat, for 

 instance, Langley (1878) showed that the chorda and sympathetic nerves both 

 give very much the same kind of saliva. The sympathetic nerve contains 

 fibres which cause great constriction of the arterioles of the gland, while the 

 chorda contains fibres which dilate them, so that it has been held (see Langley, 

 1898, p. 529) that the restricted blood supply is responsible for the relatively 

 concentrated saliva produced by the "trophic" nerve fibres, and that there is 

 no necessity to assume the existence of two kinds of nerve fibres presiding over 

 secretion. The question seems, however, to be definitely decided in the latter 

 sense by the experiments of Babkin (1913), who investigated the properties 

 of the saliva secreted reflexly by placing in the mouth, in the one case, meat 

 powder, in another case, hydrochloric acid. It was found that the blood flow 

 through the gland was equally accelerated by both, but, while the content in 

 inorganic salts was identical, the organic constituents of the saliva secreted 

 under the stimulus of meat were in four to five times as great an amount as 

 in that formed under hydrochloric acid. Since the removal of the superior 

 cervical ganglion, by which the influence of the sympathetic fibres is removed, 

 had no effect on the result, it is necessary to assume that the chorda tympani 



