The following details have been selected from the copious table of instances contained in Mr. Simon's essay, they show conclusively that the gland does not attain its greatest size for some time after birth, and that after a va riable period it gradually again diminishes. Thus in the dog, at birth the gland weighs 4'75 grs. ; from 3.1 months to 14 year after it varies from 360 to 780 grs.; from 3 to 4 years it varies from 150 to 46 grs. In the cat, at birth its weight=61 grs. ; from 19 to 37 days after it 30 to 44 grs. ; 4 to 6 years after= 20 to 3 grs. In the human fetus of 7 months the gland weighed 33 grs. ; at 8 months 40 grs.; at birth 84 to 240 grs. ; 9 months after, 270 grs.; at 21 years 40 grs. The weight of the thymus is subject to considerable varieties, which pro bably depend, as Mr. Simon points out, partly on original differences, some individuals hav ing naturally a larger proportion of thymic structure than others ; partly also on tempo rary alterations in the activity of the nutrient processes, as is well exhibited in the effect of over-exertion on the thymus of lambs re marked by Mr. Gulliver ; the size of the gland is known also to diminish when the develop ment of the muscular system is promoted, it being found to waste away much more rapidly in young oxen used for draught, than in others not so employed.
The general conclusion, which the able physiologist from whose work I have drawn so largely adopts, is, I think, truly judicious and accurate ; he estimates the period, during which the thymus persists and is active, not so much according to the space of time which has elapsed, but according to the state of the general functions of the frame : if the assimi lating processes are active and vigorous, and the supply abundant, and the demand only mo derate, the gland will be large and will persist long ; if on the contrary the first processes of nu trition are imperfectly supplied, or if great mus cular exertion creates a considerable demand, then the thymus ceases earlier to discharge its function and becomes atrophied, because the conditions no longer exist which are favoura ble to its subsistence. Now it is obvious that
in almost every individual these circumstances which so greatly affect the nutrition of the thymus may vary exceedingly, and it is there fore impossible to state an exact numerical age as the period of the highest development of the gland ; a physiological age may how ever with much certainty be named, and it is, as Mr. Simon states, "the age of early growth." The date of the earliest appearance of the thymus in the human foetus is still little more than matter of conjecture, it has not been po sitively detected before about the 9th week, when it is quite distinct to the naked eye, consisting of two lateral elongated portions lying parallel to each other on the upper part of the pericardium. Its structure at this time is distinctly tubulo-vesicular, but there is doubtless an earlier stage, when it corres ponds exactly to the simple primary tube dis covered, as before mentioned, in very early mammalian embryos. The epoch of its entirely vanishing is very variable and uncertain " about puberty it seems in most cases to suffer its chief loss of substance, and to be re duced to a vestigiary form ;" but for several years later, even up to 20 or 25, distinct rem nants may still be discovered of its structure amid the areolar tissue of the mediastinum.