DR. H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
1095 
follicles ; the changes in the blood vessels ; and the final disappearance of the gland and 
its replacement by fat and connective-tissue. 
The involution of the gland, though to the naked eye it appears to take place at a 
definite period of the gland’s existence, does not really do so, but is a very gradual 
process; for we must consider the formation of epithelioid cells and of granular cells, 
as the primary indication of involution, and this first takes place during foetal life. 
Again, the giant cells and the concentric corpuscles also aid in involution, and they are 
present during the middle and latter end of foetal life, and during the period of the 
growth of the organ ; while the formation of fibrous-tissue takes place during quite the 
latter end of growth, and during all the period of decrease in size. Further, various 
animals of the same species differ greatly, in accordance with the activity of their life, 
and the amount of food they have taken. And again, various parts of the same 
thymus differ considerably ; thus in the Calf, the upper part of the thymus in the neck 
undergoes involution some time before the lower part. 
. The formation of fibrous-tissue within the follicle. 
We have already stated that the granular cells become transformed directly into 
fibrous-tissue (see p. 1087). This transformation also takes place in the thymus of old 
animals. If we study a section of the medullary portion of the thymus of an Ox 
from six to eight years old, we find, among the lymphoid cells, and to a great extent 
replacing them, bundles of interlacing fibrous-tissue, with a considerable number of 
granular cells attached to their edges. If the thymus of a very old animal be examined, 
the whole of the medulla is seen to be transformed into fibrous-tissue and blood vessels, 
few granular cells remaining. 
The giant cells also appear, to some extent, to take part in the formation of fibrous- 
tissue, their long processes being in some instances transformed into it. 
In the medulla, the fibrous-tissue is formed also by connective-tissue-corpuscles. 
They increase in size, and become spindle-shaped (see Plate 93, fig. 79), and these 
spindle-shaped cells are then fibrillated, and become directly transformed into fibrous- 
tissue (see Plate 93, fig. 79, f and 78, /). 
The vessels of the medulla have often a very thick adventitia of newly-formed con¬ 
nective-tissue. This seems to be formed in two ways: partly by the action of the 
granular cells, and partly by the proliferation of the pre-existing adventitia of the vessel ; 
such a vessel is shown in Plate 93, fig. 81. The vessel is seen to be surrounded by newly- 
formed connective-tissue. In the lower part of the figure, fibrous-tissue is attached to 
the vessel. The large veins surrounding the medulla have often a very thick covering 
of fibrous-tissue, and in shaking the specimens roughly, it sometimes happens that the 
whole of the medulla shakes out, and leaves a ring formed of trabeculae of fibrous- 
tissue and vessels, the ring surrounding an apparent central cavity. 
In the cortex, very few granular cells or giant cells are present; in the majority of 
specimens not one can be found ; it is therefore clear that any formation of connective- 
MDCCCLXXXII. 7 A 
