1092 



THYMUS GLAND. 



Fig. 729. 



Fig, 



Diagram of fully developed ihymus. 

 Showing how the primary central tube is covered 

 and concealed by the lateral developements, each of 

 which constitutes a conical mass, a, b, c, d, with a 

 very wide base. {After Simon.) 



unbulging wall, but I have seen it distinctly at 

 a period somewhat later, when the process of 

 lateral extension had but recently commenced. 

 This was in the embryo of a sheep, not more 

 than two inches long, where the thymic cavi- 

 ties were bounded by a well-marked limitary 

 membrane, and filled with nuclei. At the 

 extremity of the cervical portion, the develope- 

 ment of bulging offsets was much less ad- 

 vanced than towards the middle of the gland, 

 so that here the central tube was very appa- 

 rent, terminating by a closed extremity, and 

 having its margins rendered irregular and 

 wavy by the vesicles which had begun to 

 rise from it {fig. 730.). The developing or- 

 gan was formed in a nidus of homogeneo- 

 fibrous tissue, interspersed with nuclei, which 

 was seen stretching across between the promi- 

 nent convexities of the bulgings. At the end 

 of the cervical portion this tissue was more 

 abundant, and there was seen running into it 

 a prolongation of the central cavity, which 

 appeared exactly like a short tube, pushing 

 on in a straight direction, and not expanding 

 nto a vesicular cavity. In a young chicken 

 the condition of the thymus was very similar, 

 and the central cavity was larger than the 

 small lateral offsets. These details, though in- 

 complete, leave scarcely a shadow of doubt that 

 Mr. Simon's account is perfectly correct, that 

 the central cavity is the primary part from which 

 the vesicular offsets successively develope 

 themselves. This central cavity may, I am 

 inclined to believe, in some cases disappear 

 more or less completely ; at least, in an em- 

 bryonic sheep, three inches long, it not only 

 bore a smaller proportion to the multiplied 

 offsets, but its wall no longer exhibited the 

 investing limitary tissue, and it seemed as if it 



Extremity of cervical portion of thymus from embry- 

 onic sheep. 2 3 in. long. 



The end of the tube is closed, the central cavity 

 is larger, the walls present numerous follicular pro- 

 trusions of irregular size and form. Towards the 

 middle the follicles are very much more developed. 



were in some measure diminishing, and losing 

 its original distinctness. From what I have 

 seen in the sheep, I should be led to think that 

 the cervical and thoracic portions of the thy- 

 mus had, in that animal at least, distinct pri- 

 mary tubes as centres of developement, so 

 completely independent have the two parts 

 seemed to be of each other. 



Mature structure of the gland. The results 

 of minute scrutiny into the structure of the 

 fully-developed . thymus, accord well with 

 those arrived at by other modes of inquiry. 

 Its surface, when freed from investing areolar 

 tissue, exhibits, though in a ralner coarser 

 manner, the minutely-divided appearance so 

 characteristic of the conglomerate glands, and 

 this is especially evident when fat cells have 

 formed in the interstices of the lobules, pat- 

 terning the surface over with a network of 

 white streaks. In a thin section taken from 

 the gland and prepared for the microscope, 

 the outlines of the vesicular cavities are rea- 

 dily seen ; they are much larger than those of 

 the salivary glands, and vary very much in 

 size ; in a human foetus, at about the mid- 

 period, they averaged ^ inch, in a calf about 

 -^g- inch, in a young guinea-pig they varied 

 from ^ to ^ inch. Their form is oval or 

 spherical, their outline distinct for about two- 

 thirds of their circumference, but in the 

 remaining part blended with adjacent ones, so 

 that there is never seen any thing resembling 



