STRUCTURE OF THE THYMUS GLAND. 369 



are completely filled.* The very wide spaces charged with 

 lymph cells, which immediately invest the follicles, are in com- 

 munication, by means of finer vessels, with the central parts of 

 the follicles. M. His regards these spaces as lymphatics ; but, 

 according to my observations, it must still remain doubtful 

 whether they are lymphatic vessels or sinuses investing the 

 follicles. 



According to the older views, f the follicles are hollow vesicles 

 invested externally by a structureless membrane, and internally 

 by a layer of connective tissue, their cavities all communi- 

 cating with a common central canal. 



Jendrassik J has demonstrated that the elementary parts of 

 the thymus gland are solid lymph follicles, in the central part of 

 which a cavity is formed by softening. I find that these 

 cavities only occur in the follicles of the thymus in man and 

 the calf, and not always even there. The central part of the 

 follicle, which, both in man and the calf, consists of a network 

 of cells with interspersed lymph corpuscles, after prolonged 

 hardening, easily becomes detached during manipulation. 



In regard to the physiological atrophy of the thymus, it con- 

 sists, according to His, of a gradual breaking-down and infiltra- 

 tion of the glandular tissue with fat, which extends gradually 

 from the septa and the surface of the follicles towards their 

 interior ; but even in the earliest period, when there can be no 

 question of atrophy, small isolated groups of fat cells may be 

 found in the investing sheaths of the follicles. 



* Kolliker, Gewebelehre, p. 485. 



f J. Simon, A Physiological Essay on the Thymus Gland. London, 1845. 

 4to. Gerlach, Gewebelehre Mainz, 8vo, Lieferung, 2 and 3. Ecker, loc. cit. 



J Anatomische Untersuchungen iiber den Bau der Thymusdruse, Sitzungs- 

 berichte der k. Akad. zu Wien., 1856, Juli-heft. 



