What is the function of secretin?

Explanation

### Step-by-Step Solution:

1. **Introduction to Secretin**: Secretin is a hormone that was discovered by Ernst Stirling in 1902. It is notable for being the first hormone to be identified.

2. **Nature of Secretin**: Secretin is classified as a protein hormone. Hormones can be broadly categorized into two types: protein (or peptide) hormones and steroid hormones. Secretin falls under the protein category.

3. **Function of Secretin**: The primary function of secretin is to stimulate the secretion of water and bicarbonate ions in the pancreatic juice. This is crucial for digestive processes.

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