The appearance of the gland in the foetus about the middle of utero-gestation is precisely similar to that of the salivary gland in the same ; it is beautifully lobulated, and sur rounded by an atmosphere of nascent areolar tissue. In the more perfectly formed con dition it is surrounded by an envelope of coarse cellular membrane, which penetrates the intervals of its larger divisions, unites the right and left portions together, and forms a general envelope, by which it is connected to the surrounding parts.
In examining the structure of the thymus, we are conducted by two eminent guides to conclusions almost identical, though by dif ferent modes of proceeding. This coincidence is of great value, and we can scarcely enter tain a moment's doubt of its being founded on real truth ; it may therefore be well to notice separately the modes of investigation above referred to.
Sir A. Cooper, by skilful manipulation, suc ceeded in unravelling the gland, and showed each lateral part to be composed of a 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. 722.). The central cavity forms a 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 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 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 (seefig. 724.); this opi 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 portions have smooth and undulated margins; and it is just the parts of the gland corre sponding to the above points, which ulti mately attain the greatest magnitude. The
third stage of developement consists in the ramification of the follicles which have budded out from the central cavity; — they do not usually elongate much, before they throw off fresh offsets, and these are completely sessile, so that they have the appearance of vesicles or imperfect spheres grouped together : the mode in which the primary offset divides is either dichotomous or quaternary (figs. 727. and 728.), probably also often with some degree of irregularity and inequality in the size of contiguous offsets. By the extension out of the primary tube does not occur simul taneously at every part, or uniformly, but chiefly at those situations which are ulti mately to attain the largest size ; thus in the foetal calf we find tolerably well developed bulgings of the primary tube opposite the angle of the jaw, the upper part of the trachea, and the pericardium, while the intervening of this follicular growth to all parts of the primary tube, and by successive lateral rami fications, occurring, as we have seen, to a greater extent in some parts than in others, the gland attains its mature size and complex structure. In this state however it consists, in very great measure, of vesicular cavities, which cluster around and completely obscure the primary tube from which they have origi nated ; yet this primary tube or reservoir does exist, and is capable, as we have seen, of being demonstrated, so that the term which Mr. Simon has proposed, as expressing the type of the mature structure, viz. tubulo vescicular, is sufficiently correct (fig. 729.). Two varieties have been observed in the seeond stage of the process ; one is, that "the tube sometimes bulges uniformly in its whole cir cumference for some extent, forming a very distinct ampulla ;" the other, " that in parts where there are yet no bulgings, it is some times flexuous, or even contorted." The observations now detailed respecting the progressive developement of the thymus, are so important in the elucidation of its structure, that I thought it very desirable to repeat them, if possible, and confirm their ac curacy by independent testimony. I have not, however, been able to procure a foetus at a sufficiently early period to discover the primary tube of the thymus, with its smooth 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 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.