THYMUS GLAND
The thymus gland presents much resemblance in structure to other glandular organs, and is classified under the head of the ductless glands, from its possessing no excretory duct.
The thymus gland is a temporary organ, attaining its full size at the end of the second year, when it ceases to grow, and gradually dwindles, until, at puberty, it has almost disappeared. If examined when its growth is most active, it will be found to consist of two lateral lobes, placed in close contact along the middle line, situated partly in the anterior mediastinum, partly in the neck, and extending from the fourth costal cartilage upwards, as high as tho lower border of the thyroid gland. It is covered by the sternum, and by the origins of the Sterno-hyoid and Sterno-thyroid muscles. In the mediastinum, it rests upon the pericardium, being separated from the arch of the aorta and great vessels, by the thoracic fascia. In the neck, it lies on the front and sides of the trachea, behind the Sterno-hyoid and Sterno-thyroid muscles. The two lobes generally differ in size: they are occasionally united, so as to form a single mass; and sometimes separated by an intermediate lobe. The thymus is of a pinkish-grey colour, soft, and lobu-latcd on its surfaces. It is about two inches in length, one and half in width, below, and about three or four lines in thickness. At birth, it weighs about half an ounce.
Structure. Each lateral lobe is composed of numerous lobules, held together by delicate areolar tissue; the entire gland being enclosed in an investing capsule of a similar, but denser structure. The primary lobules vary in size from a pin's head to a small pea. Each lobule contains in its interior a small cavity, which is surrounded with smaller or secondary lobules, which are also hollow within. The cavities of the secondary and primary lobules communicate; those of the latter opening into the great central cavity, or reservoir of the thymus, which extends through the entire length of each lateral half of the gland. The central cavity is lined by a vascular membrane, which is prolonged into all tho subordinate cavities, and contains a milk-white fluid resembling chyle.
If tho investing capsule and vessels, as well as the areolar tissue connecting the lobules, are removed from the surface of either lateral lobe, it will be seen that the central cavity is folded upon itself, and admits of being drawn out into a lengthened tubular cord, around which the primary lobules are attached in a spiral manner, like knots upon a rope. Such is the condition of the organ at an early period of its development; for Mr. Simon has shown, that the primitive form of the thymus is a linear tube, from which, as its development proceeds, lateral diverticula lead outwards, the tube ultimately becoming obscure, from its surface being covered with numerous lobules.
According to Oesterlen and Mr. Simon, the cavities in the secondary lobules are surrounded by rounded saccular dilatations or vesicles, which open into it. These vesicles are formed of a homogeneous membrane, enclosed in a dense capillary plexus.
The whitish fluid contained in the vesicles and central cavity of tho thymus, contains numerous dotted corpuscles, similar to those found in the chyle. The corpuscles are flattened circular discs, measuring about 1/1000 of an inch in diameter.
Chemical Composition. The solid animal constituents of the thymus are albumen and fibrine in large quantities, gelatine, and other animal matters. The salts are alkaline and earthy phosphates, with chloride of potassium. It contains about 80 per cent of water.
The arteries supplying the thymus are derived from the internal mammary, and from the superior and inferior thyroid.
The veins terminate in the left vena innominata, and in the thyroid veins.
The lymphatics are of large size, arise in the substance of the gland, and aro said to terminate in the internal jugular vein. Sir A. Cooper considered that these vessels carried into the blood the secretion formed in the substance of tho thymus.
The nerves are exceedingly minute; they are derived from the pneumogastric and sympathetic. Branches from the descendens noni and phrenic reach the investing capsule, but do not penetrate into the substance of the gland.
