1080 
DR. H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
The very same process by which the thymus is separated into follicles, and by which 
the follicles are multiplied in number, is also at work during involution ; only then, 
although the connective-tissue penetrates further into the follicles, and the blood 
vessels enlarge, the follicles do not increase in size; on the contrary, they diminish, 
and thus the follicles are split up and surrounded by connective-tissue (see Plate 86, 
fig- Id); and the connective-tissue increasing, the follicles are finally left as islands 
imbedded in it (see Plate 84, fig. 4). 
On examination of Plate 83, figs. 1 and 2, and Plate 84, figs. 3 and 4 (representing 
tissues treated exactly in the same manner and magnified to the same number of 
diameters) which are made specially to point out the appearances of the follicles at 
various stages of development and involution, it will be seen that the foregoing 
description is quite correct; for while in Plate 83, fig. 1, the ingrowing processes of 
connective tissue are quite fine, in Plate 83, fig. 2, the cortex is divided by broader 
bands. These bands in Plate 84, fig. 3, are still larger, and the cortical portions are 
still further separated from one another; and it will be also seen that the follicles 
have, during the process of involution, become considerably smaller ; and this in two 
ways, one of which is only indicated in the drawings, namely, by the invasion of 
the medulla by connective-tissue, while the other is clearly shown, by the great 
shrinking both in breadth and thickness of the cortex. It will be seen in Plate 84, 
fig. 4, that finally the medulla becomes converted almost entirely into connective-tissue, 
and the cortex shrinks to thin lines of tissue. 
In Mammalia this connective-tissue surrounding the follicles, and even that pene¬ 
trating them, becomes, during involution, a bed for the deposit of fat cells, as will 
be described further on (see Plate 83, fig. 2 ; and Plate 84, figs. 3 and 4); but this 
deposit of fat does not occur in the other classes of Vertebrates, and therefore should 
not be insisted on as a main factor in the involution of the follicle, as has been done 
by so many authors; indeed, in the Bird, where the thymus disappears so early, there 
is no formation of fat. 
The cortical part of the follicle is, as has been already stated, not differentiated from 
the rest of the follicle at an early period of the existence of the organ (see Plate 87, 
fig. 23); and when first differentiated in the foetal thymus is not of very great extent 
(see Plate 86, fig. 11). In the fully-developed thymus (a3 in Plate 83, fig. 1) the 
cortex is seen to be at least twice the size of the medulla. On the other hand, as the 
follicle undergoes involution, the cortex shrinks more rapidly than the medulla (see 
Plate 84, fig. 3). The cortical part of the follicle consists in a very large measure of 
lymphoid cells, and it will be shown afterwards that these cells leave the thymus by 
the lymphatics. Although the medulla contains some lymphoid cells, it also contains 
other elements not peculiar to the thymus (or the lymphoid tissues), as the granular 
cells, giant cells, and fibrous tissue. 
Briefly stated, the cortex is much larger in the fully-developed organ, and under¬ 
goes involution more rapidly than the medulla. It is composed almost entirely of 
