Thymus Gland, a glandular body situated in the lower part of the neck. The thymus reaches its maximum development shortly after birth, and from the second year of life onwards gradually atrophies, so that in adult life it is a mere remnant of the original structure. The thymus consists of a number of follicles composed mainly of adenoid tissue. The function of the gland is not well understood, but it appears likely that it is concerned in early life in the development of red blood corpuscles. In hibernating animals the gland does not atrophy, but persists throughout life, and undergoes enlargement just before each period of hibernation.