XX.] 



THE THYMUS GLAND. 



241 



THE THYMUS GLAND. 



This gland is very large in the embryo and infant, but it begins 

 a retrograde development about the sixth year, and is eventually 

 replaced by fat and connective tissue. In the rabbit it retains its 

 structure. It is composed of a number of lobes, and these again of 

 smaller lobules. A capsule composed of connective tissue holds 

 all together, and sends in septa carrying blood-vessels and lym- 

 phatics between the lobes and lobules, and also fine prolongations 

 into the interior of the latter. There are no smooth muscular fibres 

 in the septa. Each lobule consists of a cortical and a medullary 

 part. Within each lobule is a delicate network of reticular con- 

 nective tissue, finer and more like adenoid tissue in the medulla. 

 It appears to consist of branching cells, and is coarser in the cortical 

 part. The meshes are crowded 

 with leucocytes, which, however, 

 arc most abundant in the cortex. 

 The medullary substance contains 

 the concentric corpuscles, like 

 nests of concentrically-arranged 

 flattened epithelial cells (fig. 228). 

 The blood-vessels run along the 

 septa and form a capillary plexus 

 within the lobules. 



7. Thymus Gland. Harden 

 the thymus of a young animal 

 or child in Mtiller's fluid (three 

 weeks), and then in gradually- 

 increasing strengths of alcohol, 

 mounted in balsam. 



(a.} (L) Observe the capsule sending septa between the larger 

 lobules, and finer septa into the lobules, thus subdividing them into 

 smaller secondary lobules. Each such small lobule is about i mm. 

 in diameter, and as one is exactly like the others, it suffices to 

 study one. 



(b.) A darker, denser peripheral zone, the cortex, and a more 

 open light central part or medulla, the former surrounding the latter 

 (fig. 228). 



(.) (H) The septa consist of fibrous tissue with some elastic 

 fibres, with numerous blood-vessels and slits ; the latter are the 

 lymphatics. The lobule consists of adenoid tissue the mesh-work 

 not visible because it is crowded with leucocytes. 



22 Q 



FIG 



Section of a Few Lobules of a 

 Child's Thyraus. C. Cortical, M. Medul- 

 lary part; c. Concentric corpuscles, 



X 20. 



Sections stained with logwood are 



