Thymus
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Two lobes of the thymus of an adult chicken (HE): overview
The thymus in birds is a paired organ, located on both sides of the neck between the third cervical vertebra and the thyroid region. It is closely associated with the vena jugularis and the nervus vagus.
The thymus is derived from the third and fourth pharyngeal pouch together with the parathyroid gland. Tissue from one organ sometimes can occur in the other.
The thymus is divided in lobes (chicken: 6-8; duck and pigeon: 5-6). These lobes (lobi thymici) have their maximum size at young age but undergo age-dependent involution and ultimately disappear in old birds.