Step 1: Understanding the Concept:
Glands are organs that secrete substances for use in the body or for ejection from the body. They are categorized based on size and function.
Step 2: Detailed Explanation:
1. Liver: It weighs about 1.2 to 1.5 kg in an adult. It is both the largest gland and the largest internal organ in the human body.
2. Functions: It performs hundreds of functions, including bile production (exocrine), synthesis of plasma proteins, and detoxification of metabolic wastes.
3. Pancreas: The second largest gland, but significantly smaller than the liver.
4. Thyroid: The largest purely endocrine gland, but overall much smaller.
5. Spleen: Not a gland; it is a lymphatic organ.
Step 3: Final Answer:
By mass and volume, the liver is the largest glandular structure in the human body.