1096 
DE. H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
tissue cannot here arise from them, and in the cortex the network of connective-tissue- 
corpuscles is directly transformed into connective-tissue (see Plate 93, fig. 80). The 
network is thickened, until the breadth of the threads is as great as or greater than 
that of the meshes; at the same time the adventitia of the vessel enlarges. These 
changes are gradually continued, until we obtain, lastly, a few lymphoid corpuscles, 
shut in by bands of newly-formed tissue, in which later on fibrillse often appear. 
Growth of the interfollicular tissue, and invasion of this tissue into the follicle. 
Plasma cells and formation of fat. 
An important factor in involution is the growth of the connective-tissue between 
the follicles (see Plate 83, fig. 2; and Plate 84, figs. 3 and 4). In this tissue there is a 
deposition of plasma cells (see Plate 86, fig. 13*). Each plasma cell has a considerable 
zone of protoplasm surrounding its nucleus; this protoplasm contains granules, somewhat 
resembling the granules of haemoglobin ; they, however, stain very differently, and are 
not composed of haemoglobin or of fat. The cells stain in a very characteristic manner 
in indigo-carmine and carmine (see Plate 86, fig. 13), the nucleus staining deep pink 
and the granules of a blue colour; the haemoglobin of the coloured blood corpuscles 
in the preparation is green. These cells exactly resemble, both in staining properties 
and general appearance, many of the cells met with in the medulla of bone. They are 
evidently here the forerunners of fat cells.t By this growth of interfollicular- 
connective-tissue and fat, involution is probably assisted, the follicle being compressed. 
It has been stated (see p. 1079) that some of the blood vessels, surrounded to a slight 
extent by connective-tissue, pass from the outside of the follicle, between the portions 
of cortical tissue, to reach the medulla; and it is found that these vessels, with their 
surrounding connective-tissue, in their passage through the cortical-tissue, are sepa¬ 
rated from it by the capsule of the follicle, the vessels piercing the capsule to enter the 
medulla. These vessels, as they enter the medulla, have at this point a perivascular 
sheath, i.e., where they are surrounded by the endothelium of the capsu]e. The 
connective-tissue seems to press the capsule further in on the vessel, so that we get 
finger-like projections of the interfollicular-tissue, with an extension of the perivascular 
space. It is the extension of this perivascular space along the vessels, with the invasion 
of the interfollicular-connective-tissue into the follicle, which is of such importance in 
involution. 
These projections of connective-tissue gradually widen and invade the follicle, 
* For an account of the views of various authors concerning plasma cells, see p. 1074. 
f In the thymus stained by dahlia, after the manner of Ehrlich (79), cells staining deeply with the 
dahlia are met with in the centre of the follicle (see Plate 86, fig. 12). The nucleus becomes purple, 
and the granules of the same colour, but of a much deeper tint. The cells are often near the granular 
cells, which are themselves unacted upon by the blue solutions. They are of the same size as those met 
with in the interfollicular tissue. It is difficult to state the significance or function of these deeply 
staining cells, the method of Ehrlich not being very satisfactory except as a method of detection. 
