Final answer:
The tiny finger-like projections in the lining of the duodenum are called villi, which are covered with even smaller projections called microvilli, increasing the surface area for nutrient absorption.
Step-by-step explanation:
The lining of the duodenum is composed of not only thousands but millions of tiny finger-like projections known as villi. These structures play a crucial role in the digestion and absorption of nutrients. The surface of these villi is covered with even smaller projections called microvilli, which form what is referred to as the brush border. These microvilli greatly increase the surface area of the plasma membrane, enhancing the absorption of food nutrients into the body. Furthermore, the microvilli are equipped with enzymes responsible for the final steps in digesting carbohydrates and proteins.