DR. H. WATNEY ON THE MINUTE ANATOMY OE THE THYMUS. 
1091 
corpuscles are sometimes of considerable size ; and here, also, the capsule is finally 
converted into fibrous tissue. The concentric corpuscles are not found when the 
thymus is first formed, but can be seen during the middle and later periods of foetal 
life (see Plate 90, figs. 49 and 50). They are most numerous during the first period 
of involution, e.g., in the Calf, when the animal is from six to eighteen months old; a 
few are found even in very old animals, as in the Ox, twenty years old. 
It is important for us to understand the relations of the concentric corpuscles to the 
blood vessels, since Cornil and Ranvier (50) and Afanassiew (51 and 33) have con¬ 
cluded that these bodies are formed from the endothelium of the vessels. It is true 
that they are attached to the vessels, as has been noticed by His (46) and other 
observers ; the connexion of the concentric corpuscles to the network ensures their 
connexion to the vessels; but they have a more intimate connexion where the capsule 
of epithelioid cells has, in its growth, surrounded a vessel. This has partially taken 
place in Plate 91, fig. 63, v, and completely in Plate 92, fig. 69. It has already been 
mentioned that in that part of the medulla where the granular cells and concentric 
corpuscles are formed there are only a few vessels, and these are very fine. In injected 
specimens the vessels may be seen to pass through the large compound concentric 
corpuscles in every direction but one, never through the central opaque masses. This 
is well shown in Plate 85, fig. 7. The very large compound concentric corpuscle fills 
nearly one-third of the medulla; the central part of the mass has fallen out, but the 
remainder shows five concentric bodies. The blood vessels are found in the outer part 
of the compound concentric corpuscle, but do not enter the granular masses, although 
they may wind around them as at d. I have two other preparations which show 
very similar appearances. 
Various views have been held as to the origin of the concentric corpuscles (see p. 1067). 
It appears to me that there can be no doubt but that they arise from the epithelioid 
and granular cells ; the question therefore resolves itself into this—from what do the 
epithelioid and granular cells arise ? I think I have shown in pages 1084 and 1087, 
that they take origin from connective-tissue-corpuscles. Another source which might be 
suggested, which has not been already discussed (see page 1087) would be the remains 
of epithelial cells, of which the thymus is supposed (by some authors) to be at 
first formed; this is the view of His (52) and Stieda (53). They, however, do not 
attempt to support their view by evidence, although Stieda says that the concentric 
corpuscles cannot be injected ; Afanassiew (33) showed however that the injection 
penetrates the concentric corpuscles, and it has been shown in the preceding paragraph, 
and in Plate 85, fig. 7, that vessels undoubtedly do penetrate into these corpuscles. 
The proposition that the concentric corpuscles arise from the remains of the epi¬ 
thelial cells presents some probability, because of the undoubted epithelioid character 
of the cells found in the medulla of the thymus, and of those taking part in the 
formation of the concentric corpuscles. This similarity to epithelial cells, however, is 
met with in other places, as in pathological new formations. Thus epithelioid cells 
