THE THYMUS GLAND. 1125 



found to consist of two lateral lobes placed in close contact along the middle line, 

 situated partly in the superior mediastinum, partly in the neck, and extending 

 from the fourth costal cartilage upward as high as the lower border of the 

 thyroid gland. It is covered by the sternum and by the origins of the Sterno- 

 hyoid and Sterno-thyroid muscles. Below, it rests upon the pericardium, being 

 separated from the arch of the aorta and great vessels by a layer of 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 



FIG. 714. 1. Upper portion of the thymus of a foetal pig of 2" in length, showing the bud-like lobuli and 

 glandular elements. 2. Cells of the thymus, mostly from a man. a. Free nuclei, b. Small cells, c. Larger. 

 d. Larger, with oil-globules, from the ox. e,f. Cells completely filled with fat, at / without a nucleus, g, h. 

 Concentric bodies, g. An encapsulated nucleated cell. h. A composite structure of a similar nature. 



occasionally united so as to form a single mass, and sometimes separated by an 

 intermediate lobe. The thymus is of a pinkish-gray color, soft, and lobulated on its 

 surfaces. It is about two inches in length, one and a half in breadth 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, and are made up of a number of small nodules or follicles 

 which are irregular in shape and are more or less fused together, especially 

 toward the interior of the gland. According to AVatney. each follicle consists of 

 a medullary and cortical portion, which differ in many essential particulars from 

 each other. The cortical portion is mainly composed of lymphoid cells supported 

 by a delicate reticulum. In addition to this reticulum, of which traces only are 

 found in the medullary portion, there is also a network of finely-branched cells 

 which is continuous with a similar network in the medullary portion. This 

 network forms an adventitia to the blood-vessels. In the medullary portion there: 

 are but few lymphoid cells, but there are, especially toward the centre, granular* 

 cells and concentric corpuscles. The granular cells are rounded or flask-shaped 

 masses attached (often by fibrillated extremities) to blood-vessels and to newly- 

 formed connective tissue. The concentric corpuscles are composed of a central 

 mass consisting of one or more granular cells, and of a capsule which is formed 

 of epithelioid cells which are continuous with the branched cells forming the 

 network mentioned above. 



Each follicle is surrounded by a capillary plexus from which vessels pass into 

 the interior and radiate from the periphery toward the centre, and form a second 

 zone just within the margin of the medullary portion. In the centre of the medulla 

 there are verv few vessels, and they are of minute size. 



