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Your NCERT chapter on chemical coordination says, in one line, that secretin acts on the exocrine pancreas and stimulates the secretion of water and bicarbonate ions. Behind that line is the experiment that started endocrinology. It is also one of the clearest examples you will meet of how a well-designed control settles an argument. This post walks through what Bayliss and Starling did, the numbers in their paper, and how to reason through it in an exam.
What was the question in 1902?
By 1902 everyone agreed that acid chyme entering the duodenum made the pancreas pour out juice. The question was how the signal travelled. In their paper The mechanism of pancreatic secretion (Journal of Physiology, 1902), Bayliss and Starling, working at University College London, summarise the leading view. Ivan Pavlov and his pupils had shown that the flow begins when chyme enters the duodenum, mainly because of its acidity. Pavlov regarded this secretion as reflex, carried by nerves from the gut lining to the pancreas.
If you have read our post on Ivan Pavlov and conditioned reflexes, you know how strong his evidence for nervous control of digestion was. Bayliss and Starling set out to find the nerve pathway, and instead found something else.
What happened in the "crucial experiment"?
The paper gives the date and details. On 16 January 1902 they worked on an anaesthetised dog of about 6 kg. They removed the nerve masses around the main arteries to the gut, cut both vagus nerves, and then took a loop of jejunum, tied it off at both ends, and cut every nerve going to it. In their words, the loop was connected to the body "merely by its arteries and veins." A tube in the pancreatic duct counted drops of juice.
20 c.c. of 0.4% hydrochloric acid put into the duodenum gave about 1 drop every 20 seconds for some 6 minutes. That only confirmed earlier work.
Then 10 c.c. of the same acid put into the nerve-free jejunal loop produced "a similar and equally well-marked effect." The authors call this "the turning-point of the whole research."
With no nerves left, the signal had to travel in the blood. But earlier workers had shown that acid injected straight into the blood did not excite the pancreas. So the acid had to be making the gut lining release something.
The next step was quick. They scraped the lining from the loop, ground it with sand and 0.4% acid, filtered it and injected the extract into a vein. After a delay of about 70 seconds, the pancreas secreted at more than twice the rate seen at the start. They named the substance secretin.
What else did they find in the same experiment?
Two more results show careful thinking. First, they boiled the extract and it still worked, so secretin was not an enzyme, since enzymes are destroyed by boiling. Second, an extract from the last 6 inches of the ileum had no effect on the pancreas, which matched earlier findings that acid placed low in the small intestine does not trigger secretion. The substance came from the upper intestine.
Worked example: reading the numbers
Questions built on this experiment usually test logic, but you can practise with the paper's own figures.
Rate in the duodenum test: 1 drop every 20 s. Over about 6 minutes, that is 6 × 60 = 360 s, and 360 ÷ 20 = 18 drops.
The vein injection gave "more than twice" that starting rate, so more than 2 drops every 20 s, or more than 6 drops per minute.
The 70-second delay before secretion began is worth noticing. A delay of over a minute is consistent with a substance that has to be carried round the body in the blood before it reaches the pancreas.
Common mistakes:
Writing that secretin makes the pancreas release enzymes. NCERT says it stimulates water and bicarbonate; cholecystokinin (CCK) stimulates pancreatic enzymes.
Saying Bayliss and Starling disproved all nervous control of digestion. They showed this response could happen without nerves. Nervous control of other secretions still exists.
Forgetting the boiling test when asked how they knew secretin was not an enzyme.
Where does the word "hormone" come from?
Starling gave the Croonian Lectures at the Royal College of Physicians in London in June 1905, published in The Lancet in August 190511877-5). Describing substances that travel in the blood to coordinate distant organs, he called them chemical messengers, or "hormones", from a Greek verb meaning "I excite or arouse". Your NCERT Class 11 Biology Chapter 19 still begins from that idea before giving the modern definition of hormones as non-nutrient chemicals that act as intercellular messengers.
For how hormone levels are then kept steady, see our post on hormonal feedback loops.
What can a student take from Bayliss and Starling's way of working?
Remove one explanation at a time. Cutting the nerves did not prove a hormone existed; it removed the nerve explanation and left the blood as the only path.
Follow up straight away. The extract, the boiling test and the ileum test were all done in the same experiment. When a result surprises you, ask the next question while the set-up is in front of you.
Respect the view you are testing. Their paper sets out Pavlov's work fairly before showing where it did not hold. Good answers do the same.
If the four gut hormones and their targets keep swapping places in your head, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can check one hormone at a time.
Next step: draw the dog experiment as a flow chart, mark where the nerves were cut, and write one line on what each step ruled out.
