414 MANUAL OF HISTOLOGY. 



work. They generally pass in as far as the medullary portion 

 or central cavity, and form a ring about this. 



The central canal, or band of the thymus, is lined with a 

 vascular membrane, and communicates with the central cavi- 

 ties of the lobules and follicles. Along its interior the bulg- 

 ings of attached vesicles, or groups of the same, may be seen. 



Many authorities consider that the central cavities in the 

 follicles and lobules are produced artificially by the break- 

 ing down of the very delicate tissue. There is much proba- 

 bility that this is the case. More investigation, however, is 

 needed, and meanwhile the cavities and canals are described 

 as above, since it is extremely rare to find a human thymus in 

 which they do not appear to exist, no matter how careful the 

 preparation. 



The blood-vessels. The thymus is not a very vascular 

 gland. Its arteries are distributed in the capsule and along 

 the central band. From these parts they pass unaccompanied, 

 as a rule, by the veins, to the interlobular tissue, and are dis- 

 tributed to the follicles, as has been described. 



The lymphatics accompany the blood-vessels along the 

 central band. From there it is stated (His) that they pass to 

 the interlobular tissue and are distributed around the follicles, 

 communicating by minute channels with the centre of the fol- 

 licle. This latter point, however, lacks confirmation. 



Development. The thymus gland is found in all vertebrates 

 except amphioxus (Huxley). It is developed, like the lym- 

 phatic glands, from the mesoblastic layer, and can be seen 

 early in foetal life. It appears first as a closed tube, which is 

 probably (Quain) a mass of embryonic cells enclosed in a mem- 

 branous capsule. Along this projections bud out which are 

 gradually transformed into lobules. By the twelfth week it 

 has become well developed. 



The thymus is an organ of foetal and infant life only. It 

 grows rapidly until the second year, when it begins to undergo 

 a fatty degeneration and atrophy. By the seventh or eighth 

 year it is a small, fatty mass. This degeneration of the thy- 

 mus takes place in all the animals which have the gland. 



