THE THYIdUS GLAND 



1443 



sist 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 

 level of the fourth costal cartilage upward as high as the lower border of the thy- 

 roid gland. It is covered by the sternum and by the origins of the Sternohvo'id 

 and Sternothyroid 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 

 t lies on the front and sides of the trachea, behind the Sternohyoid and Sterno- 

 thvroid muscles. The two lobes generally differ in size; they are occasionally 

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

 The thymus is of a pinkish-gray color, is soft, and is lobulated on its sur- 

 faces. It is about two inches (5 cm.) in length, one and a half inches (3.75 cm.) 

 in breadth below, and about a quarter of an inch (6 mm.) in thickness. At birth 

 it weighs about half an ounce. 



FIG. 1198. A lobule of the thymus of a child, as seen under low power. C. 

 Cortex, c. Concentric corpuscles within medulla. 6. Bloodvessels, tr. Tra- 

 beeuhe. (Schiifer.) 



FIG. 1199. Elements 

 of the thymus. X 300. 

 a. Lymph corpuscles. 6. 

 Concentric corpuscle. 

 (Schiifer, after Cadiot.) 



Structure (Figs. 1198 and 1200). 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 which are irregular in shape and 

 are more or less fused together, especially toward the interior of the gland. Each lobule consists 

 of a cortical and medullary portion, which differ in many essential particulars from each other. 

 The cortical portion is mainly composed of dense lymphoid tissue, consisting chiefly of Ivrnpho- 

 cytes and hyaline 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 bloodvessels. The medullary portion consists of diffuse lymphoid tissue 

 (granular cells) and concentric corpuscles (corpuscles of Hassal). The granular cells are 

 rounded or flask-shaped masses attached (often by fibrillated extremities) to bloodvessels 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 concentrically 

 arranged epithelioid cells which seem to be continuous with the branched cells forming the net- 

 work mentioned above. 



Each lobule 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 very few vessels, and 

 they are of minute size. 



Watney has made the important observation that hemoglobin is found in the thymus either 

 in cysts or in cells situated near to or forming part of the concentric corpuscles. This hemo- 

 globin varies from granules to masses exactly resembling colored blood corpuscles, oval in the 

 bird, reptile, and fish; circular in all mammals except in the camel. Dr. Watney has also dis- 

 covered in the lymph issuing from the thymus similar cells to those found in the gland, and, 

 like them, containing hemoglobin either in the form of granules or masses. From these facts 



