The thymus gland (Gr. [Greek: thymos], from a fancied resemblance to the
corymbs of the Thyme) is a light pink gland, consisting of two unequal
lobes, which lies in the superior and anterior mediastina of the thorax
in front of the pericardium and great vessels; it also extends up into
the root of the neck to within a short distance of the thyroid gland. It
continues to grow until the second year of life, after which it remains
stationary until puberty, when it usually degenerates rapidly. The
writer has seen it perfectly well developed in a man between 40 and 50,
though such cases are rare; probably, however, some patches of its
tissue remain all through life. Each lobe is divided into a large number
of lobules divided by areolar tissue, and each of these, under the
microscope, is seen to consist of a cortical and medullary part. The
cortex is composed of lymphoid tissue and resembles the structure of a
lymphatic gland (see LYMPHATIC SYSTEM); it is imperfectly divided into a
number of follicles. In the medulla the lymphoid cells are fewer, and
nests of epithelial cells are found, called the concentric corpuscles of
Hassall. The vascular supply is derived from all the vessels in the
neighbourhood, the lymphatics are very large and numerous, but the
nerves, which come from the sympathetic and vagus, are few and small. H.
Watney (_Phil. Trans._, 1882) has discovered haemoglobin, and apparently
developing red blood corpuscles, in the thymus. (For further details see
Gray's or Quain's _Anatomy_.)
_Embryology._--The thymus is formed from a diverticulum, on each side,
from the entoderm lining the third branchial groove, but the connexion
with the pharynx is soon lost. The lymphoid cells and concentric
corpuscles are probably the derivatives of the original cells lining
the diverticulum.
_Comparative Anatomy._--The thymus is always a paired gland. In most
fishes it rises from the dorsal part of all five branchial clefts; in
Lepidosiren (Dipnoi), from all except the first; in Urodela from 3rd,
4th and 5th, and in Anura from the 2nd only (see T.H. Bryce,
"Development of Thymus in Lepidosiren," _Journ. Anat. and Phys._ vol.
40, p. 91). In all fishes, including the Dipnoi (mud fish) it is
placed dorsally to the gill arches on each side. In the Amphibia it is
found close to the articulation of the mandible. In the Reptilia it is
situated by the side of the carotid artery; but in young crocodiles it
is lobulated and extends all along the neck, as it does in birds,
lying close to the side of the oesophagus. In Mammals the Marsupials
are remarkable for having a well-developed cervical as well as
thoracic thymus (J. Symington, _J. Anat. and Phys._ vol. 32, p. 278).
In some of the lower mammals the gland does not disappear as early as
it does in man. The thymus of the calf is popularly known as "the
chest sweetbread."