The thymus gland is a structure of very variable dimensions in different subjects; its size appears to depend to some extent on the general nutrition, which is greater in well-nourished infants than in those who are emaciated. It attains it maximum development at the end of the second year, after which it slowly diminishes in size till puberty, when rapid fatty degeneration takes place and the organ practically disappears, becoming infiltrated with connective tissue, in which, however, remains of the gland may here and there be found.
It consists of two lobes of unequal size, separated by a mesial fissure, anil lies in the superior and anterior mediastinal spaces, and partly in the neck. It extends down to about the fourth costal cartilage, and comes into relation anteriorly with tho sternum and costal cartilages, laterally with the pleura? and lungs, and posteriorly with the pericardium, trachea, and vessels of the neck. In specimens hardened in tilu by formalin the deep surface of the gland is found to show a concavity for the pericardium and deep grooves for the pulmonary artery, innominate veins, and superior cava (Cunningham). The two loin’s of the thymus ascend in front of the trachea to an unequal distanco, usually, however, falling short of the thyroid gland, and extending outwards as far as tho carotid arteries. Tho blood-supply is from the inferior thyroid and internal mammary arteries; the nerves of the gland are derived from the vagus and sympathetic.
Tho thymus is a ductless gland, and consists a number of lobules separated from one another by connective-tissuo septa arising from its sheath. In the clusters of lymphoid tissue which compose the lobules, cortical and medullary layers can he distinguished. In the latter 1 tho connective-tissuo matrix is coarser and the i lymphoid cells fewer; and in the medulla, also, the characteristic thymus structures—Hassails corpuscles—are placed. These consist of concentric layers of flattened epithelial cells surrounding a granular nuclear corpuscle.
The thymus develops about the seventh week of fa-tal life as a tubular diverticulum from the third visceral cleft. The walls of the diverticulum are composed of epithelium which proliferates rapidly, and as tho diverticulum grows downwards it becomes expanded laterally in its lower parts Finally, tho connection between the gland and the pharyngeal cleft from which it arose is severed, and the epithelial structure is invaded and almost entirely replaced by adenoid and connective tissue, the only remnant of its original constituents left in the mature gland being the Hassall’s corpuscles already referred to.
Functions of the Thymus Gland In Human Body
Little is known as to the function of the thymus. In all probability it is the same as that of tho lymphatic tissues elsewhere, the thymus juice containing numerous leukocytes which find their way into the general circulation through tho veins of the gland. From the fact that it is usually persistent and enlarged iu cases of Graves’ disease,and sometimes in acromegaly and myxcedema, the idea suggests itself that it may have some function (an internal secretion 1) correlated with that of other ductless glands. Svehla found that the intravenous injectiou of thymus extract produces (in dogs) a great fall in the blood pressure and acceleration of the heart, and, in lethal doses, dyspnwa and collapse. But in reality little is known as to the physiology of the organ.
Disk-asks ok the Th rut’s.
The thymus is for the most part inaccessible to the ordinary methods of physical examination, though in young infants a little dulncss may be made out over the upper part of the sternum as low as the third rib. The majority of the morbid changes to which tho organ is liable are, either cause of their rarity or because they are insusceptible of clinical diagnosis, of pathological interest alone. Only one condition, in fact,— hypertrophy of the thymus,—is of any practical importance; see Status Lymphatics ; the other diseases of the organ may be dealt with very briefly.
1. Tuberculosis is on the whole rare.
It may occur as a primary lesion, but is more commonly part of a generalized tuberculosis. lac-obi found it thrice in children under one year, in each case as a part of a generalized familiar tuberculosis.
In cases of hereditary syphilis the usual vascular lesions and increase of connective tissue are sometimes found. Small multiple abscesses are also said to occur, but there is som doubt as to whether these are really of an inflammatory nature, as the secretion of tho thymus gland bears considerable resemblance to pus.