THE THYMUS, 187 



entire thymus or only through separate segments of it, and this 

 in cases where no preparation of any kind or injection had 

 been used. I look upon the occurrence of a narrow central 

 canal, as the original and usual condition, but believe, that in 

 certain cases it may be expanded by a more abundant forma- 

 tion of the secretion, and ultimately be converted into a large 

 cavity. 



§ 183. 



Intimate structure of the thymus. — When a lobule is stripped 

 of its investing coat, consisting of common connective tissue, with 

 fine elastic fibres of the finer sort Fi 244 



and frequently with scattered fat- ^ ^ - \ 



cells, its external surface, fissured 

 in correspondence with the separate 

 gland-granules, comes into view. 

 Under strong magnifying powers, 

 there is now presented a very Sk'^H B t 



thin (0-0005 — 0-001' ), indistinctly 

 fibrous, or almost homogeneous 

 membrane, quite correctly de- 

 scribed by Simon, which is con- :, --^tkYU' 

 tinuous over an entire lobule or V \ fc\p 

 even the whole gland and must be 

 placed in the same category with 

 the wall of the follicles in Peyer's 

 patches, the tonsils, &c. Within --- 

 this envelope, between it and the cavity of the lobule, lies a 

 greyish white, soft, easily lacerable substance, \ — jf' thick, which, 

 when examined microscopically, appears to consist of nothing but 

 free nuclei and minute cells, and on this account has, by agree- 

 ment of all observers, hitherto been regarded as the secretion of 

 the supposed gland- vesicles. But this substance cannot be washed 

 away, which would have been the case had it lain loosely in the 

 space enclosed by the delicate membrane ; on the contrary, it 



Fig. 244. Transverse section through the summit of an injected lobule of the 

 thymus in a Child, x 30 diam.: a, membrane of the lobule; b, membrane of the 

 gland-granules ; c, cavity of the lobule from which the larger vessels branch out 

 into the granules, on the surface of which they terminate, occasionally forming 



