Which of the following is NOT a major endocrine gland?

The muscle tissue does not qualify as a major endocrine gland because it primarily functions in movement and structural support within the body rather than in hormone production and regulation. Endocrine glands are specialized organs that secrete hormones directly into the bloodstream, playing crucial roles in regulating various physiological processes, such as metabolism, growth, and reproduction.

In contrast, glands such as the thymus, adrenal glands, and thyroid are all vital components of the endocrine system. The thymus is involved in immune function and T-cell maturation, the adrenal glands produce hormones like cortisol and adrenaline in response to stress, and the thyroid regulates metabolism through the secretion of thyroid hormones. Each of these glands plays a distinct and essential role in maintaining homeostasis within the body.