THE THYMUS GLAND. 



591 



of 



Fig. 418. 



cells are much less numerous than the free nuclei, and from 

 to T2 Vo f an i ncn i 1 * diameter; the latter being 5 cW to 

 an inch. 



The arteries (Fig. 418) are sent from the external surface through 

 to the internal cavity, and there ramify in a delicate expansion of 

 areolar tissue lining it. From this 

 arterial plexus, branches enter the ca- 

 vity of each lolule, and form a capillary 

 plexus in their external portion, or 

 the gland-corpuscles, entirely filling 

 them, but never extending further than 

 to the inner surface of the homogeneous 

 membrane investing them. The fibres 

 above mentioned support the capilla- 

 ries just described, and require no spe- 

 cial description. The lymphatics are 

 numerous"; and nerves accompany the 

 arteries, though not yet traced to their 

 terminations. 



The cavities of the thymus inclose a 

 grayish- white or milky, faintly acid, 

 albuminous fluid, containing numerous 

 nuclei, isolated cells, and sometimes 

 concentric corpuscles, next to be de- 

 scribed. 



Between the ages of 12 and 20 

 years, involution of the thymus com- 

 mences. During this, peculiar spheri- 

 cal bodies are found in the substance of the lobules, called the con- 

 centric corpuscles. These (first noticed by Hassall and Virchow) 

 are: 1. Simple, ^Vir to T2\nj f an i ncn in diameter, with a* thick 

 concentrically striated membrane and a granular substance within, 

 appearing sometimes as a nucleus, at others as a cell; 2. Compound, 

 -S^TT to lis f an i ncn i Q diameter, and consisting of several simple 

 corpuscles inclosed in a common laminated envelop. By the 40th 

 year, the thymus is usually entirely removed. 



The function of the thymus is not certainly known. Mr. Simon 

 considers it " a sinking fund in the service of respiration." It is 

 developed by two tubular offsets from the larynx, containing blas- 

 tema. It is not stationary after birth, as sometimes stated, but grows 



Transverse section of an injected lob- 

 ule of the thymus of a child, a. Mem- 

 brane of the lobule, b. Membrane of 

 the gland-corpuscles, c. Cavity of the 

 lobule from which the larger vessels 

 branch out into the corpuscles, on the 

 surface of which they terminate, occa- 

 sionally forming loops. (30 diameters.) 



