1098 
DR. H. WATNEY ON THE MINUTE ANATOMY OE THE THYMUS. 
during involution, there are considerable differences in the thymus of different animals. 
Thus, in the Bird and the Tortoise the concentric corpuscles are often enlarged, and 
transformed into small cysts. In the Dog the cysts are lined in great part (as we have 
already seen) with ciliated epithelium. In Mammals, fat invades the follicle ; while in 
the other classes of Vertebrates none is found. 
Thymus of the Bird. 
The thymus of the Fowl, the Turkey, and the Pelican have been examined. 
In the Bird, the thymus is attached by connective-tissue to the vein and the nerve 
of the neck; so that in cutting longitudinal sections, the follicles are found to be 
attached to a central string, composed of fat, some small amount of connective-tissue, 
and sometimes including the large vein and the nerve. In the Chicken of five or six 
weeks old, the microscopical characters of the gland, as regards both cortex and medulla, 
are very similar to those of the fully-formed organ in Mammals. In involution, which 
takes place at an early period of life in the Bird, the distinction between cortex and 
medulla is lost, as the cortical portion soon disappears, and the great number of 
concentric corpuscles gives the appearance of numerous medullary portions in the 
follicle. 
The concentric corpuscles differ somewhat in their origin, and in their arrangement, 
from those of Mammals. They are formed principally of large epithelioid cells (see 
Plate 95, fig. 99), only a few granular cells taking part in their formation. These 
epithelioid and granular cells form masses, which are not so much compressed, nor are 
they so definitely arranged in concentric layers as in the case of Mammals (see Plate 95, 
figs. 100 and 101). They contain vacuoles of various sizes, with thickened borders. 
The vacuoles are formed and increase by the coalescence of the vacuoles of the 
individual epithelioid and granular cells. (Compare Plate 95, figs. 99, 100, and 101.) 
Within them, we often meet with granular masses and masses of haemoglobin. On 
teasing and examining the thymus, we find many vacuolated cells (see Plate 95, fig. 98). 
Sometimes the cells exist as only a small ring of nucleated protoplasm, surrounding 
the vacuole (see Plate 95, fig. 98, h"). 
A few giant cells are present (see Plate 95, fig. 97), where one is shown enclosing a 
granular mass. 
The fibrous-tissue is formed in two ways : either from the granular cells or from the 
concentric corpuscles, the edge of the vessels being thickened in involution. Fat does 
not invade the follicles, nor is the gland finally surrounded by fat. The essential 
process in involution is, however, similar to that in Mammals ; the points of contrast 
are : the rapid diminution of the cortex, the large number of epithelioid cells, and the 
large number of concentric corpuscles, the rapid involution, and the absence of invading 
connective-tissue and of fat. The gland is finally replaced by a small amount of con¬ 
nective-tissue. 
