298 THE THYMUS GLAND. 



other along- the middle line, and are nearly symmetrical in form, thoug-ii 

 generally unequal in size, sometimes the left, and at other times the 

 right lobe being the larger of the two. An inlermaliate lobe often exists 

 between, the two lateral ones, and occasionally the whole body forms a 

 single mass. 



Each lateral lobe is of an elongated triangular form. The apex usually 

 mounts up into the neck, reaching as high as to the lower border of the 

 thyroid body. The basr, rests on the upper part of the pericardiimi, to 

 which it is connected by areolar tissue. The anterior surface, shghtly 

 convex, is covered by the first and the upper part of the second piece of 

 the sternum, reaching, in the infant at birth, as low down as the level 

 of the fourth costal cartilage. It is attached to the sternum by loose 

 areolar tissue, but opposite the upper part of that bone is separated from 

 it by the origins of the sterno-hyoid and sterno-thyroid, which muscles 

 also cover it in the neck. The posterior surface, somewhat concave, 

 rests, in the thorax-, upon part of the pericardium* upon the front of the 

 aortic arch and the large arteries arising from it, and also on the left 

 innominate vein. In the neck it lies upon the front and corresponding- 

 side of the trachea. Its external border is in contact with the corre- 

 sponding layer of the mediastinal pleura, near the internal mammary 

 artery, and higher up (in the neck), with the sheath of the carotid artery. 

 The internal border is in close contact with that of the opposite lobe. 

 The dimensions of the thymus vary according to its stage of develop- 

 ment. At birtli it measures rather more than two inches in length, an 

 inch and a half in width at its lower part, and about three or four lines 

 in thickness. Its weight at that period is about half an ounce. Its 

 specific gravity, which is at first about 1-050, diminishes as the gland 

 continues to waste. 



Structure. — The lateral halves or lobes of the thymus gland are 

 each invested by a capsule of thin areolar tissue, which sends parti- 

 tions into the gland between the several lobules : on its outer surface 

 the capsule is covered by a layer of flattened epithelioid cells. Each 

 lobe consists of numerous polyhedral lobules, the most of them from 

 two to five lines in diameter, connected by a more delicate inter- 

 vening areolar tissue. These primary lobules are each made up of 

 a number of small nodules or follicles (fig. 200, b, b), as they have 

 been termed. These are, in many respects, similar in structure to 

 ordinary lymphoid follicles, such as those of the tonsils or of Peyer's 

 patches m the intestine : consisting, like these, of retiform tissue, the 

 meshes of wdiich are filled with lymph corpuscles ; at the surfiice of 

 each follicle the retiform tissue is somewhat closer, so as to form a 

 species of capsule for it. In some animals these capsules completely 

 enclose the follicles, but in others, including man, several follicles 

 may be united towards the centre of the lobule, which is then conmionly 

 of softer consistence than the other parts, and apt to break down if 

 not perfectly fresh, so as to give the deceptive appearance of a centra] 

 cavit}^ (see fig. 200). Scattered here and there in the retiform tissue arc 

 peculiar corpuscles, composed of a substance which strongly refracts the 

 light, and readily becomes stained by carmine. They present an ap- 

 pearance of concentric striation, and are known as the concentric cor- 

 2mscl('s of^ Hdssall. They vary in size from that of a blood-corpuscle 

 to three times that diameter, or more ; the larger often contain smaller 

 ones in their interior. 



