THYMUS GL AND.—(Fren ch, Le thymus; Italian, Tiny; German, Die Brustdruse ; Lat., Thymus; Greek, evpoe.)— It is proposed in this article to adopt the following arrange ment : First, to treat of the gland as it exists in the human subject, comprehending its ordi nary and structural anatomy, and its develop ment. Secondly, to give a sketch of the comparative anatomical history of the organ. Thirdly, to treat of its physiology. Fourthly, to mention what has been observed of morbid changes occurring in it.
HumAN ANATOMY.- Sir A. Cooper's de scription of the gland in the human subject is as small as to be but just perceptible. At three months (fig. 718.) its increase is in propor tion to the relative magnitude of the foetus, and thus it continues to grow gradually and equally (fig. 719.) to the seventh month, when it en larges out of proportion to its former growth. At eight months it is large, but at the ninth month (fig. 720.) has undergone a sudden change, becomes of great size, and is said to weigh half an ounce, from which circumstance, however, on account of the cavities which it contains, and the varieties to which it is sub ject, no judgment of its bulk can be formed. It increases after birth, and continues large to the first year, when it slowly disappears to the time of puberty; and in after age it ceases to have cavities, and becomes a body of very small He next notices the following varieties in configuration :—" Although the gland is usu ally double, and the one side united to the other by cellular membrane only, yet it some times happens that a third thoracic lobe exists, which appears to join one lobe with the other, but which allows, under a careful dissection, of their being separated. There are also two other varieties I have seen ; the first is the vena innominata passing through the gland, and the second, the same vein placed anteriorly to the cervical lobes. Indeed, I scarcely find two organs alike in form ; sometimes they are round, whilst others are of great length, and are so thin that the serpentine disposition of their lobes may be seen without dissection. The left gland is often larger than the right ; but even in this respect so much variety is observable, that it appears if the bulk of the whole be the same, that it is of little import ance which may be of the greater magnitude, the right or left gland, as its secretion will be equally abundant." The relative situation of the thymus gland to the adjacent parts is described as follows : — " In cutting through the sternum in its long axis, and then separating its two lateral portions, so as to give a good view of the mediastinum, the thymus gland appears situ ated behind the first and part of the second bone of the sternum ; and posteriorly to the origins of the sterno-hyoidei and sterno-thy roidei muscles. It reaches more than half
way down the sternum at birth, viz. to the fourth rib, and extends from thence into the neck near to the thyroid gland. It is con nected to the sternum and origins of the sterno-hyoidei and sterno-thyroidei muscles by cellular tissue ; it adheres strongly, by a coarse cellular membrane, to the pericardium ; an teriorly and laterally the internal mammary arteries and veins take their course. The reflection of the pleura descending from the cartilages of the ribs on each side, and con tinued to the fore part of the pericardium forming the anterior mediastinum, makes its lateral boundaries, and separates it from the lungs ; posteriorly it rests upon the vena in nominata, and upon the fascia of the thorax, which descends from the sternum and first rib to the curvature of the aorta, and to the three large vessels which spring from it." " Such, then, is the relative situation of the gland in the chest. In the dissection of the cervical portion of the thymus, the platysma myoides and external jugular vein are first turned aside, and the origins of the sterno-mastoidei muscles are raised; when this has been accom plished, the sterno-hyoidei appear covering and passing over the thymus gland. The sterno-thyroidei muscles ... cover this organ anteriorly ; but when they are removed, the cervical portions of the thymus are seen on the anterior and lateral parts of the trachea, and just below the thyroid gland, where it passes on the fascia on the fore part of the air tube, and unites with the larynx by liga ment." " The internal jugular veins are placed an teriorly and laterally to the cervical portion, and the carotid arteries, with the par vagum, appear more externally." " The first bone of the sternum and sternal ends of the clavicle cover the junction of the cervical with the thoracic portion of this gland." " In many of the subjects which I have ex amined, the cervical portion of the thymus passes higher upon the right than on the left side, and I have generally seen it joined by a ligament to the larynx, and by vessels to the thyroid gland." In a human foetus, at about the fourth-and a-half month, I found the thymus consisting of two lateral portions, of which the right was the larger (in another of similar age the left was) ; this portion extended downward, lying upon the pericardium, as far as opposite the right auricular appendix, and reached upwards only to the left brachio•cephalic vein, which it did not cross ; the left extended downwards, over the pericardium, to a point opposite the middle of the trunk of the pulmonary artery, and passed up, lying upon the vena transversa, and afterwards upon the side of the trachea, be tween it and the common carotid, till it arrived at the level of the bifurcation of the arteria innominata.