THYMUS GLAND. 



1099 



cess we here find occurring the superaddition 

 of a thymus gland, its first appearance in 

 the ascending scale of organization." Among 

 the Percnnibranchiata it is very interesting to 

 observe, that the gland is gradually suppressed 

 in proportion as the respiration becomes more 

 completely aquatic, a thymus is found in the 

 Menopoma, Amphiuma, Axolotl, and Meno- 

 branchus, but not in the Siren or Proteus ; its 

 position is rather peculiar, it lies in the neck, 

 on each side, along the lateral aspect of the 

 spine, just behind that prolongation of mucous 

 membrane which unites the branchial cavity 

 to the pharynx. 



Pisces. In fishes Mr. Simon has been unable 

 to discover any trace of a thymus, after search- 

 ing, carefully, in more than twenty genera ; 

 this result accords well with the absence of 

 the gland in the fish-like batrachian larva, and 

 with its disappearance in the lowest Perenni- 

 branchiata. 



The following are the conclusions which 

 the eminent physiologist, so often referred to, 

 deduces from a survey of his detailed and 

 elaborate investigation. (1.) The presence 

 of the thymus gland is co-extensive with pul- 

 monary respiration. (2.) Its shape and po- 

 sition are variable and unimportant. (3.) Its 

 size and duration are, generally speaking, in 

 proportion to the habitual or periodical inac- 

 tivity of the animal. (4.) Where it remains 

 as a persistent organ, it is usually but one of 

 several means for the accumulation of nutri- 

 tive material ; its continuance, under such 

 circumstances is generally accompanied 

 though in some instances superseded by a 

 peculiar accessory contrivance, the fat body. 



PHYSIOLOGY. The time is now past when 

 an eminent physiologist could declare of the 

 organs usually known as glands without ducts, 

 "that in regard of their intimate structure and 

 physiological meaning, they are all equally 

 and utterly unknown to us." With respect 

 to their structure, it may, I think, be said, that 

 they are as well understood, or perhaps even 

 better than the true glands, being in fact less 

 complex, and their constituent parts less in- 

 dependent. Their physiological meaning, it 

 must be confessed, is still obscure ; yet even on 

 this dark part of the demesne of our science 

 light is beginning to break, which we may fairly 

 hope will continue to brighten. 



The results afforded by examination of the 

 thymus in the lower animals seem certainly to 

 connect the gland most closely with pulmo- 

 nary organs of respiration, and it would, 

 therefore, seem a natural conclusion, that it 

 subserves some purpose which has to do with 

 the aeration of the blood. It is also found 

 that the size of the gland may vary in a short 

 space of time very considerably, that is, that 

 its contents are capable of being absorbed very 

 quickly, as proved by the fact mentioned by 

 Mr. Gulliver, that in over-driven lambs, the 

 thymus will soon shrink remarkably, and be 

 nearly drained of its contents, but will be- 

 come as quickly distended again during rest 

 and plentiful nourishment. This seems to 



imply that the material necessary for the 

 supply of the respiratory process is furnished 

 by the thymus, which thus, in Mr. Simon's 

 words " fulfils its use as a sinking-fund in 

 the service of respiration." The persistence 

 of the gland, moreover, is observed to vary 

 considerably according to the muscular ac- 

 tivity of the animal, thus it disappears quickly 

 in young oxen put to the plough, it endures 

 longer in animals of quiet habits than in the 

 restless and energetic beasts of prey, and 

 vanishes at a very early period in the class of 

 birds, while it persists long in that of reptiles. 

 From this it seems to follow, that the use 

 fulfilled by the gland in the service of res- 

 piration is more or less superseded when the 

 muscular system is called into a high state of 

 activity, i. e. when there is considerable waste 

 of tissue yielding fuel for respiration. In 

 the hibernating animal, where the gland, pre- 

 parative to the winter sleep, is transformed 

 into a mass of fat, its application to the de- 

 mands of the respirtory process seems 

 scarcely doubtful, as the chemical nature of 

 the contents of its cavities is then peculiarly 

 appropriate to neutaralise theoxydising agency 

 of the air. 



Probable, however, as these views may ap- 

 pear, they have been confronted by the fol- 

 lowing weighty objections. An able physio- 

 logist, in the Brit, and Foreign Med. Re- 

 view, observes, that the condition of young, 

 rapidly growing animals, and that of hiber- 

 nating animals, are rather opposite than pa- 

 rellel, that whereas in the latter, the waste of 

 the tissues is reduced to a minimum, in the 

 former it is certainly greater than in the adult, 

 so that there can be no deficiency of effete 

 material to feed the respiratory furnace. The 

 demand in the young creature is for plastic 

 materials, out .of which the rapidly growing 

 and rapidly changing structures may be built 

 up and renewed. " On the other hand, in the 

 hibernating animals all the nutritive actions 

 are at zero, and the respiration for a long 

 period is entirely dependent on the stores of 

 fatty matter which have previously been set 

 apart from the food. The demand is here for 

 combustible materials." 



The writer then observes, that the che- 

 mical nature of the contents of the thymus 

 correspond so exactly at the two periods of 

 active growth and hibernation to the kind of 

 demand which must then exist in the system, 

 that he conceives it more probable that the 

 use of the gland at these times is different ; 

 in the one slowly yielding up its hydro-car- 

 bonous contents to supply fuel to the respi- 

 ratory process, in the other performing the 

 principal part in elaborating, by means of its 

 myriad nuclei, fibrine from albumen, the plastic 

 from the non-plastic element. " As the de- 

 mand for plastic material becomes less ener- 

 getic the thymus diminishes in size and dis- 

 appears, the production of plastic matter 

 within the absorbent and sangniferous vessels 

 being then sufficient for the wants of the sys- 

 tem. Or if the organ remains," its struc- 

 ture " and the nature of its function changes, 



