THE THYMUS. 593 



apparent. In no case does there exist any epithcliura lining the cavi- 

 ties and consequently, the comparison of the innermost part of their wall 

 with a mucous membrane is untenable. 



The common cavity or central canal of the thymus, presents the same 

 structure as the lobules, except that externally there is a thicker fibrous 

 layer, and internally a less thick granular stratum, with rather larger 

 vessels. In a fully developed thymus it, as well as all the secondary cavi- 

 ties, contains a grayish-white or milky, faintly acidj^itz't?, often in large 

 quantity, in which, together with a clear fluid abounding in albumen, 

 numerous nuclei, isolated cells and, under certain circumstances, also 

 concentrical corpuscles [vide infra) are contained. The lymphatics of 

 the thymus are numerous and nerves may be readily demonstrated upon 

 the arteries, although they cannot be traced as far as their terminations. 



Besides the above-described normal elements, there occur, especially 

 at the period of involution of the organ, other peculiar spherical struc- 

 tures, which, with Ecker, I would terra the concentric corpuscles of the 

 thymus. They present very various forms, which, however, it seems to 

 me may conveniently be reduced to two ; viz. 1, simple, O'OOG-0'01 of 

 a line in size, with a thick concentrically striated membrane and agranu- 

 lar substance in the interior, sometimes appearing like a nucleus, some- 

 times as a cell; and 2, compound, as much as 0-01 or even 0-08 of a 

 line in size, and consisting of several simple corpuscles, surrounded by 

 a common laminated envelop. These bodies, which were first noticed 

 by Hassall and Virchow, and were further investigated by Ecker and 

 Bruch, it appears to me arise not from a direct metamorphosis of the 

 nuclei and cells in the wall of the glandular lobule, but from the succes- 

 sive deposition of an amorphous material around them ; and conse- 

 quently that they are analogous in their mode of formation to the cor- 

 puscula amylacea of the brain, the prostatic concretions, &;c. The lami- 

 nated portion consists of a substance certainly not of a fatty nature 

 which ofi"ers considerable resistance to alkalies, approaching the colloid 

 substance and the substance of the prostatic concretions and corpuscula 

 amylacea, being probably formed from a change in the albumen in the 

 glandular walls. The situation of these concentric corpuscles is ex- 

 ternal to the secretion of the thymus, and principally in the innermost 

 part of the glandular parietes, where the larger vessels occur.* 



§ 184. Development of the Thymus. — According to Remak, the thymus 

 of the Chicken originates in the separation, by constriction, of the bor- 

 ders of the last two (third and fourth) branchial fissures, which arc lined 



* [Wliatever may be the true nature of the Hassallian corpuscles, tlicy do not seem to be 

 an essential element of the thymus or its secretion, as they are not found in that body in 

 Fishes (Skate, Ray, Sturgeon, Zeus faber) ; although present in the Reptilia. Leydig, '• Anat. 

 histol. Untersuchung. Q. Fische iind Reptilien," pp. 21 and GG. — Tks.] 



38 



