1090 
DR. H. WATNEY ON THE MINUTE ANATOMY OE THE THYMUS. 
able to perceive the tissues from which the corpuscle is formed. Nearly in the centre 
there is a granular cell lying in a vacuole, which has a thickened border, deeply stained 
by hsematoxylin; around the vacuole are epithelioid connective-tissue-corpuscles, with 
many nuclei. These cells enclose two other granular cells. 
In the upper part of Plate 92, fig. 69, another and smaller concentric corpuscle is 
shown ; the structure is essentially the same. In the centre there is a granular cell, 
lying in a vacuole with a thickened border. The vacuole is surrounded by epithelioid 
connective-tissue-corpuscles, and the neighbouring cells are very large. 
Plate 91, fig. 64, shows nearly the same ; two small concentric corpuscles are in 
process of formation; the centre of each contains a granular mass, lying in a vacuole 
with a thickened border. The simplest concentric corpuscle of all may be seen in 
Plate 89, figs. 46 E and G, and Plate 91, figs. 66 and 67, where the granular mass, or 
in fig. 66 the three granular masses, are surrounded by protoplasm with proliferating 
nuclei. 
It is thus seen that the concentric corpuscles and the giant cells arise from the same 
sources, i.e., from the granular cells, and the epithelioid connective-tissue-corpuscles. 
In considering the function of the concentric corpuscles, and their behaviour during 
involution, one point must be apparent to all, i.e., that many of them disappear; for 
if the thymus of a very old animal be examined, very few will be found, and these a.s 
a rule will be small. It is also not difficult to prove that the greater part of most of 
the concentric-corpuscles, connective-tissue threads, and fibrous processes, become 
fibrous-tissue; the threads and processes becoming thicker, in consequence of the 
deposition of new tissue, by the granular cells, attached to their sides and lying 
between them. In these bundles of connective-tissue we find vessels which are 
either the pre-existing vessels enclosed by the newly-formed connective-tissue, or newly- 
formed vessels. Finally, the concentric corpuscles and the fibrous bands are changed 
into connective-tissue containing large vessels, which generally have one or two peri¬ 
vascular membranes. These changes are most easily traced in the thymus of the Calf 
and of the Child, 
We shall find in some animals that the concentric corpuscles are transformed into 
cysts; this is notably the case in the Dog, Bird, and Tortoise. 
Great differences will be noticed in sections of the thymus of different animals, both 
in the number and in the size of the concentric corpuscles. In the Child they are very 
numerous, and are often large and compound. In the Kitten they are still larger. In 
the Calf, on the other hand, they are small, and comparatively few in number. As a 
rule, the very large concentric corpuscles are compound ; the layers of epithelioid cells, 
which in the single concentric corpuscle form only a capsule, are here extended over a 
large area, and embrace two or three, or even five concentric corpuscles (see Plate 85, 
fig. 7). At times we meet with abortive concentric corpuscles, where the capsule has 
been formed; but the central parts are composed of unchanged lymphoid cells of the 
connective-tissue network, and of traces of the reticulum. These abortive concentric 
