1090 THYMUS GLAND, 



rope, on which the lobes and lobuli are set 



somewhat like the beads on the string of a 

 necklace (fig. 721.). By injecting also the 

 glandular cavities with some fluid, as alcohol, 

 capable of hardening the tissue, or with co- 

 loured gelatine, which sets and permanently 

 distends them, he demonstrated the existence 

 of a central cavity or reservoir, communicating 

 with the glandular cavities by orifices leading 

 into pouches situated at the roots of the 

 lobes (fig. 122.). The central cavity forms a 



Fig. 722. 



Section of thymus showing the reservoir, cells, and 

 pouches. (After Sir A. Cooper.) 



general communication between the different 

 lobes ; it does not maintain a straight course, 

 but passes in a somewhat spiral manner, be- 

 ginning from the lower part of the thoracic 

 portion, and extending even into the extremity 

 of the cervical part of the gland : its size varies 

 in different parts, being largest near the centre 

 of the thoracic, and least at the communication 

 of the thoracic with the cervical, part of the 

 gland. Sir A. Cooper conceived the reservoirs 

 to be lined by a very vascular mucous mem- 

 brane of somewhat villous character, but this 

 does not appear in reality to exist. 



Such were the principal results obtained by 

 a most skilful and eminent anatomist, with all 

 the appliances and aids that his science could 

 at that day supply ; they were truly valuable 

 facts, but not so " luciferous," not so exhibit- 

 ant of physiological meaning, as those ob- 

 tained by a subsequent inquirer, who, availing 

 himself of the more penetrating ken of the 

 modern achromatic lens, and seeking rather to 

 learn from the instructive examples which 

 Nature herself sets forth, than from results of 

 his own devising and producing, has both 

 confirmed the conclusions drawn from a less 

 refined scrutiny, and invested them with a 

 more correct bearing and interpretation. I 

 refer, of course, to the admirable researches 

 of Mr. Simon, which I now proceed to detail, 

 respecting the structure of the thymus, as 

 illustrated by its developement. 



Developement. The first trace of the or- 



gan which has been discovered is in the 

 form of an exceedingly delicate tube, lying 

 along the carotid vessels in the neck, not 

 straight, but wavy at one part, and termi- 

 nating by closed extremities at both ends 

 (fig. 723.). Its wall is formed by a transpa- 



Fig. 723. 



Primary tube. (After Simon.') 



rent homogeneous tunic, marked at regu- 

 lar intervals with elongated thickenings (the 

 remnants probably of the nuclei of primor- 

 dial cells), and enclosing granular matter, but 

 no distinct corpuscles. There seems some 

 probability that this tubular form, though 

 found to prevail in very early embryos, may 

 not be the really primitive one, but that a 

 linear series of cells is first developed, which 

 are afterwards blended together by fusion, so 

 as to constitute a tube (see fig. 724.); this opi- 



Fig. 724. 





Supposed origin of primary tube. (After Simon.) 



nion however, I suspect, will not be confirmed ; 

 the limitary membrane of glandular and other 

 structures has generally appeared to me to be 

 produced quite independently of cells, so far, 

 at least, as that it should be regarded identical 

 with their coalesced envelopes. In the next 

 stage of developement, the homogenous wall 

 of the tube begins to bulge, and swell out into 

 vesicular cavities, which at first have wide 

 communications with the central canal or 

 tube, and are quite sessile, but afterwards 

 become attached by short and rather narrow 



