DR. H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
1079 
The follicles, except in earliest embryonic life, are composed of two parts, a cortex 
and medulla, which, as we shall see, differ very materially in structure. The difference 
between the two parts can be seen by the naked eye in a thymus which has been left 
for two or three weeks in bichromate of potash or monochromate of ammonia. In 
sections made from hardened tissues and stained in haematoxylin, this difference is 
easily seen (see Plate 83, figs. 1, 2; and Plate 84, fig. 3), and depends on two main 
facts: the first, that the concentric corpuscles, the giant cells, the granular cells, the 
numerous endothelial connective-tissue-corpuscles (together with the large vessels and 
fibrous-tissue met with in certain states of involution) are found, with few exceptions, 
in the medulla, and that therefore there are necessarily much fewer lymphoid corpuscles 
in that part of the follicle ; the second fact is, that the reticulum is but rarely met with 
in the medulla, while it surrounds nearly every lymphoid corpuscle in the cortex; and 
the preponderating presence of these two deeply staining bodies, the lymphoid corpuscles 
and the nodal points of the reticulum, causes the cortex to assume a much darker and 
bluer colour in preparations stained by logwood. In some follicles there are seen to be 
two or even three medullary portions (see Plate 83, fig. 1, c). 
The follicles are sometimes united by cortical-tissue (see Plate 83, figs. 1 and 2, a), 
and at other times by medullary-tissue (see Plate 83, figs. 1, 2 ; and Plate 84, fig. 3, b). 
These follicles, when united by medullary-tissue, may sometimes be spoken of as com¬ 
pound follicles (see Plate 84, fig. 3, b). 
It will be noticed that processes of the connective-tissue penetrate the follicle in a 
radiating manner (see Plate 83, figs. 1 and 2, c't'). These processes often contain 
somewhat large vessels (see Plate 85, fig. 6, V'), and thus the medulla is in contact 
with the surrounding connective-tissue and vessels; and it is not necessary for the 
vessels to pass through the cortical lymphoid tissue in order to reach the medulla. 
In the embryonic state, the upper part of each half of the thymus may be said to 
consist of one follicle (see Plate 87, fig. 23). This is not at first differentiated into 
cortex and medulla, and the differentiation does not occur until the follicle is divided 
up by connective-tissue, as will now be described. The blood vessels penetrating into 
the follicle from the outside, pass in, surrounded by connective-tissue ; and by the 
ingrowth of this connective-tissue, the original follicle is divided into a considerable 
number of follicles (see Plate 86, fig. 11).* The follicles increase in size, and are each 
divided, and thus the thymus is formed of a large number of follicles, partially or 
wholly separated from one another ; these follicles forming lobules, which are often 
united. During the ingrowth of this connective-tissue, the blood vessels increase in 
size. This process goes on during the whole growth of the thymus, and the follicles 
increase in size and in number. Although the connective-tissue is constantly ingrowing, 
the bands of connective-tissue, in the fully-developed thymus, are very thin (see 
Plate 83, fig. 1). 
* This description applies to the development of the thymus, except in its very earliest stage; this will 
he described under the head of “ Development.” 
MDCCCLXXXII. 6 Y 
