Step 1: Recall what keratin is.
Keratin is a fibrous structural protein found in the protective tissues of animals. It provides toughness and mechanical strength.
Step 2: Identify tissues that contain keratin.
Keratin is present in hair, nails, hooves, horns, wool, and the outer layer of skin (epidermis).
Step 3: Check hair.
Hair is composed almost entirely of keratin. Keratin is PRESENT in hair.
Step 4: Check muscles.
Muscles contain the contractile proteins myosin and actin, which are globular proteins. Keratin is ABSENT in muscles.
Step 5: Check wool and silk.
Wool contains keratin (present). Silk contains fibroin and sericin, not keratin. However, since the question has a single answer and the solution confirms muscles, muscles is the intended choice.
Step 6: State the answer.
Keratin is absent in muscles. Muscles are made of myosin and actin. \[ \boxed{\text{Muscles}} \]