THYMUS GLAND. 



1095 



which are united and concentrated in front of 

 the heart, by passing from behind forwards in 

 harmony with corresponding motions of the 

 neighbouring part, becomes the thymus. The 

 structure of these organs is identical with that 

 of the blastoderma (?). Their probable func- 

 tion, namely, to prepare by the action of their 

 nucleated cells, and to throw into the vascular 

 system a matter necessary for the nutrition of 

 the animal during the period of its active 

 growth, a function which the observations and 

 opinions of the majority of physiologists have 

 assigned to them, is also essentially the same 

 with that of the blastoderma." 



Development of Size. Having now examined 

 the anatomy of the thymus, and traced its 

 development with a view to the exact elucida- 

 tion of its structure, we have next to follow 

 out the successive periods of its growth, in 

 order to determine whether it be an organ 

 having special relation to fcetal or to extra- 

 uterine life. The former alternative was that 

 to which the older anatomists, and even Sir 

 Astley Cooper, inclined ; but the correctness 

 of the latter seems now abundantly established. 

 Meckel, Hewson, Cloquet, and Sir A. Cooper 

 himself, all concur in stating that it continues 

 to grow at least up to the end of the first year 

 after birth ; and more recently the evidence 

 accumulated by Mr. Simon from his own ob- 

 servations and those of Hangsted have quite 

 set this important point at rest. 



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^ months to 1^ 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=6^ grs. ; from 19 to 37 days after it= 

 30 to 44 grs.; 4 to 6 years after =20 to 3 grs. In 

 the human foetus of 7 months the gland 

 weighed 33 grs. ; at 8 months 40 grs. ; at birth 

 84 to 210 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, "theageof 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. 



COMPARATIVE ANATOMY. In presenting a 

 sketch of the comparative anatomy of the 

 thymus, I can but follow the elaborate ac- 

 count given by Mr. Simon. 



Mammalia. Among the Quadrumana the 

 thymus has, in the more anthropoid Apes, 

 nearly the same general shape and relations 

 as in the human subject, the cervical portion 

 seems to be variously developed in different 

 genera. 



Among Cheiroptera the gland seems to be 

 persistent in the genera Vespertilio and Ga- 

 leopithecus, at least so far as anatomical in- 

 quiry has yet proceeded ; it consists of a 

 thoracic portion embracing the base of the 

 heart, and two cornua ascending parallel to 

 each other on either side of the windpipe 

 In a Bat which I dissected on the 20th of 

 March, and which was then in a wakeful 

 state, I could find no organ which I could 

 positively conclude to be a thymus. On each 

 side, however, of the root of the neck there 

 existed a pretty large yellowish lobulated 

 mass, resembling a good deal the aspect of a 

 conglomerate gland. It consisted of conical 

 lobes which were bounded and defined by a 

 distinct homogeneous membrane exactly re- 

 sembling the limitary tissue ; this was I think 

 continued by reflection from one lobe to an- 

 other, so as to form a common envelope to 



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