344 PRINCIPLES OF GENERAL PHYSIOLOGY 



are supplied ; the other is by stimulation of nerve fibres which terminate in the 

 secretory cells. It may, of course, ultimately be found that, in the actual cell 

 system itself, the processes are identical in the two cases, so that the nerve may 

 act by production of the same chemical substance which excites directly, or the 

 chemical excitant may act on the same terminal mechanism as the excitatory process 

 in the nerve fibre does. 



It has long been known that certain drugs, of which pilocarpine is the most 

 familiar, are capable of causing practically all glands to enter into activity. The 

 fact that glands, such as the mucous glands of the air passages, which do not 

 appear to be supplied with nerves, are excited seems to indicate that this effect 

 is produced independently of nerve supply. On the other hand, adrenaline causes 

 a powerful secretion of the submaxillary gland of the cat and we know that 

 the action of this substance is exerted on the endings of the sympathetic nerves, 

 wherever they are found. It appears, then, that a chemical substance may excite 

 the cells of glands either directly, or through the medium of the nerve terminations 

 on them. In the former case, it is probable that the drug may act on some 

 definite part of the cell system, the "receptive substance" of Langley (1906). 

 After administration of atropine, pilocarpine is ineffective in producing secretion, 

 nor can it be produced by exciting the nerves of such glands as are supplied with 

 them. Pilocarpine seems to be an abnormal excitant for gland cells, since its 

 action is very violent and profound morphological changes are caused in the cells. 

 In this respect it differs from a normal chemical excitant, such as secretin for the 

 pancreas, the mechanism of which we will now consider. It was shown by Pavlov 

 and his fellow- workers (1901, p. 132 of the English translation) that the presence 

 of various substances in the duodenum, especially acids, causes pancreatic juice to 

 be poured in. This excitation of the pancreas was looked upon as a reflex through 

 the nervous system until Bayliss and Starling (1902, 1), in investigating the local 

 nervous reflexes connected with the alimentary canal, found that it was still produced 

 by acid in the duodenum after all accessible nervous communications had been 

 divided. This fact suggested that some chemical mechanism was at work, set going 

 by the acid. The injection of acid into the blood current has no effect, as would be 

 expected, so that some substance must be produced by the action of the acid on the 

 mucous membrane of the intestine, which substance then diffuses into the blood 

 and, arriving at the pancreas, excites it to action. The next step was to scrape off 

 the mucous membrane and rub it up with sand and dilute hydrochloric acid. 

 After neutralisation and filtration, this extract was injected into a vein and we 

 were naturally delighted to find that a copious flow of pancreatic juice was the 

 result. It may be pointed out that it is quite immaterial whether the whole of 

 the nerves were actually cut in the previous part of the experiment, since it was 

 the belief that they were cut that led to the search for a chemical mechanism. 



Further details as to the properties of this " secretin." as we called it, being unable to think 

 of a better name, will be found in Chapter XXIV. The name itself has now come into general 

 use and, whatever objection may be made to it, it must lie admitted that it lias the advantage 

 of making no assertion as to the chemical nature of the substance, as to which we have little 

 positive knowledge. 



The juice formed under the action of secretin appears to be identical with that 

 formed during natural digestion; perhaps it may be rather more dilute; it 

 contains trypsin in the inactive, zymogen form, amylase and lipase, together with 

 alkaline salts. The pancreas can be caused to secrete continuously for many 

 hours by repeated doses and, although in the later stages a somewhat more dilute 

 juice may be obtained, it is a matter of considerable difficulty to induce signs of 

 fatigue in the cells, so far as microscopic observation can detect them. 



Atropine has no effect on the action of secretin, contrary to its action on 

 secretion produced by stimulation of nerves. Secretin must act, therefore, on the 

 cells directly, or, at all events, on some part of the cells beyond nerve terminations. 



How far this natural form of chemical stimulation of glands applies in general 

 remains as yet uncertain. It is comparatively unimportant in the case of the 

 salivary and sweat glands, but the work of Pavlov (1901, Lecture VII.) and of 

 Edkins (1906) shows that the gastric juice is partly produced by the agency of 



