Thymus
The thymus attains full size in the second year of life, at which time it ceases to grow, gradually atrophying until it disappears. When present, the gland can be found in the neck region in front of the trachea. It is an endocrine gland which secretes thymic hormone. Thymic hormone is involved in the proliferation and differentiation of white blood cells.
References
Gray, H. 1977. Gray's Anatomy. Crown Publishers, Inc. New York. 1257 pp.
Hadley, M.E. 1984. Endocrinology Prentice-Hall, Inc. New Jersey. 547 pp.