DR. H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
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corpuscles. If sections are made of the thymus of a Dog from six to nine years old, 
some of the follicles will be found to contain large cysts (see Plate 94, fig. 84); these 
cysts are lined by ciliated epithelium. In the thymus of Dogs from eight to thirty 
months old we shall find (1) small concentric corpuscles, (2) small cysts lined by epi¬ 
thelioid cells, and (3) small cysts lined by ciliated epithelium. If the thymus of a 
young Dog from one to four months old be examined, the follicles will be found, both 
as regards the cortex and the medulla, to resemble those met with in the Calf or other 
animals, and to contain small concentric corpuscles; no cysts can be found. 
We will first describe the cysts as met with in older animals (see Plate 94, fig. 84). 
The follicles are small; there is a considerable amount of fat and connective tissue 
between them, which has to some extent penetrated into them (see Plate 94, fig. 84, F). 
In the follicle, cysts are present of such size as to be visible to the naked eye. They 
contain, in most cases, a deep yellow mass, somewhat resembling extravasated blood 
(see Plate 94, fig. 84) ; although this mass never contains any coloured corpuscles, even 
in preparations where the corpuscles can be clearly seen in the neighbouring blood 
vessels. The borders of the cysts are lined, over the greater part, by ciliated epithelium ; 
in other places, apparently by a thickening of the tissues of the thymus, although it is 
possible that the cysts are lined throughout by ciliated epithelium, but that the cells 
have been lost in making the specimen. 
These epithelial cells are either columnar, as in Plate 94, fig. 91, or sub-columnar, as 
in Plate 94, fig. 90. The individual cells can be separated by teasing in saline solution, 
and are very similar to the epithelial cells of the trachea (see Plate 94, fig. 92). I 
have attempted, in five instances, to find out if any active movement of the cilia takes 
place in these cells during life, but without success, except in one case, in which the 
trachea had been accidentally opened, and where the result was of no great value. 
The chief point is to trace the origin of the cysts and of the ciliated epithelium. 
If we make sections of the thymus of a Dog from eight to sixteen months old, we 
shall find (a) quite small concentric corpuscles with small vacuoles (see Plate 94, fig. 85); 
or ( b ) small concentric corpuscles, containing cysts or vacuoles, partially filled with 
degenerated cells which often show traces of vacuolation (see Plate 94, fig. 86). 
Further, we find in Dogs between these ages (c) small cysts containing either (1) dege¬ 
nerated epithelioid cells (see Plate 94, figs. 88, dg, and 89)—the masses of degenerated 
cells show traces of vacuolation (see Plate 94, fig. 89, va), and they are at times attached 
to the sides of the cysts (see Plate 94, figs. 88 and 89);—or (2) masses of granules, 
showing only traces of cell origin (see Plate 94, fig. 87) ; or (3) masses of haemoglobin. 
The cysts are lined wholly, or in part (1), by ciliated epithelial cells ; or (2) by masses of 
protoplasm, not, apparently, completely differentiated into cells, although these masses 
bear cilia on their surface (see Plate 94, figs. 88 and 89); or (3) by epithelioid cells (see 
Plate 94, fig. 89, ep). (Compare the epithelioid cells, ep. in Plate 94, figs. 87 and 88.) 
The ciliated cells, when they are found in these smaller cysts, are sometimes sub- 
columnar, but frequently cubical or even flattened. It will further be noticed that 
