THE THYMUS GLAND. 



299 



Tis.. 200. 



According' to the description given Ijy Astley Cooper the thymiis presents a con- 

 tinuous anfractuous cavity, the extensions of wliich pass into the lobules : it is pro- 

 babel, however, that the cavity described was produced artificially, the lymphoid 

 tissue towards the centre of each lobule being rather looser than in the outer paits. 



Vessels and Nerves. — The artrri/'H of the thymus are derived from various 

 sources, viz., from the internal mammaiy. the inferior and superior thjToid, 

 the subclavian and carotid arteries. Theii' branches penetrate to the centre 

 of the lobules, whence tliey radiate outwards, terminating in capillaiy vessels, 

 which form a network within each follicle 

 (fig. 200), and pass at its exterior into the 

 'reins. These pui'sue a different course from 

 the arteries : they, for the most part, o^jen 

 into the left innominate vein.* 



The h/mphafic.s are largo. According to 

 the observations of His on the calf, the 

 larger blood-vessels passing to the centre 

 are each accompanied by two or more lym- 

 phatic trunks. These arise from an inter- 

 lobular plexus, which again is in connection 

 with vessels which suiTound and enclose 

 the individual follicles (as in the intes- 

 tinal follicles). 



The nerves are very minute. Haller 

 thought that they were jiartly derived from 

 the phrenic nerves, but. according to Cooper, 

 no filaments from these nerves go into the 

 gland, although they reach the investing 

 capsule, as does also a branch from the 

 descendens noni. Small filaments, derived 

 from the pneumo-gastric and sympathetic 

 nerves, descend, on the thyroid liody, to the 

 upper part of the thymus. Spnpathetic 

 nerves also reach the gland along its various 

 arteries. 



Development and G-rowtli. — The early 

 development of the thymus has been care- 

 fully studied by Simon, whose researches 

 were chiefly conducted in the embrj'os of 

 swine and oxen. In embryos about half 

 an inch in length, it may he distinguished 

 with the aid of the microscope : and in 

 those of one and a half inch, with a simple 

 lens. When first distinguishal)le, it appears 

 to consist of a simjile closed tube, lying 

 along the carotid vessels (most likely the fiihe 



so described is a collection of embryonic cells enclosed in a naembranous capsule). 

 It has no connection with the respiratory mucous membrane, as was supposed by 

 Arnold ; and so soon as discoverable, it is found to be perfectly distinct from 

 the thji'oid body. At intervals along the sides of this tube or capsule, small pro- 

 jections bud out. and these go on subsequently branching out into groups of two 

 or four, — the fonnation of the jiermanent follicles being merely the last repeti- 

 tion of this i^rocess. In the human foetus at the seventh week, the thjrmus is 

 bi-lobed below but still single above ; at about tlie ninth week, it consists of two 

 minute elongated parallel parts, l.^ang chiefly on the ujjper part of the pericar- 

 dium ; at the twelfth week it is already comparatively broad, and its sui-face is 

 entii'ely covered with lobules : it then increases rapidly until birth, but not with 



Fig. 200. — Transverse Section of 

 A Lobule op an In.jected Infan- 

 tile Thymus Gland (from Kol- 

 liker). Magnified. 



rt, capsule of connective tissue sur- 

 rounding the lobule ; h, h, follicles ; 

 c, cleft in the centre of the lobule, 

 pi-ol)ably produced by the shi-inking 

 away of the soft follicular substance ; 

 from it the blood-vessels are seen to 

 extend towards and ramify in the 

 spheroidal folHcles. 



* In' some animals the arterial, as well as the venous, branches are found at the 

 periphery of the follicles, to near the centre of which the capillaries converge (as in 

 Peyer's patches). 



