1098 



THYMUS GLAND. 



a quantity of oily matter as a deposit in the 

 glandular cavities. 



Fig. 732. 



Lower end of thymic tube from chick one week old. 

 Diameter at'a 1-15 in. 



Replilia. In the class of reptiles much un- 

 certainty has prevailed respecting the existence 

 of a thymus gland ; it has by some been con- 

 founded with the thyroid, by others denied to 

 exist at all. The microscope has again in the 

 same skilful hand cleared away the doubt, and 

 rendered it certain that a true thymus is found 

 in almost all reptiles, distinct from, and inde- 

 pendent of, the thyroid. In young specimens 

 of Chelonia there is found, extending upwards 

 on each side along the carotid, or between it 

 and the subclavian, an elongated white or 

 yellowish mass, possessing the characteristic 

 thymic structure, and easily distinguishable by 

 this from the true thyroid, which lies in the 

 median line between the carotids, near their 

 origin. I have repeated Mr. Simon's obser- 

 vation on a tortoise, said to be about a year 

 old, and found in the situation he indicates an 



organ, which, though having to the naked eye 

 somewhat the appearance of fat, is shown by 

 the microscope to have the characteristic 

 structure of a thymus. The lateral cavities 

 are considerably more developed than in the 

 brid, they are of very various size and some- 

 what irregular, bounded by a distinct limitary 

 membrane, and filled with well formed nu- 

 clei, togother with a quantity of opaque gra- 

 nular-looking matter consisting of minute 

 oily molecules. In Emydosauria the thoracic 

 portions of the thymus are large and of pris- 

 matic shape, they meet on the base of the 

 heart, overlapping the thyroid, and thence ex- 

 tend upwards along the carotid vessels to the 

 base of the skull where they terminate ; in 

 the lower part of the neck they diminish 

 somewhat in size, but afterwards continue of 

 uniform diameter. 



In several Saurians (Lacertldae y Geckos, 

 Chameleons) the thymus resembles that 

 just described, save that the pericardiac por- 

 tion is absent ; in adult specimens of others, 

 as Isturius and Scincidas, no thymus has. 

 been detected, but a mass of fat exists just 

 above the base of the heart, which may 

 perhaps result from a transformation of the 

 gland. 



In Ophidia the thymus observes one un- 

 deviating type of arrangement, it is found 

 lying on each side along the carotid, often 

 not strictly symmetrical, its lobes elongated 

 and sometimes broken into two or more 

 pieces. Most serpents possess a peculiar mass 

 of fat which is developed in connection with 

 the thymus, and often obscures or conceals it ; 

 in two specimens of rattlesnakes Mr. Simon 

 found this fat body was absent, but is inclined 

 to believe it exists in those which inhabit the 

 temperate latitudes. The thymus of serpents 

 is supposed by Mr. Simon to be persistent, and 

 to undergo transformation into fat. 



From the Batrachian reptiles it was natural 

 to expect, that in consequence of the remark- 

 able transformation which some of them un- 

 dergo, and the intermediate condition main- 

 tained by others between reptile and piscine 

 life, some important information would be 

 gained respecting the true relation and import 

 of the thymus. So it has proved; for in the 

 Frog only a mass of fat is found existing in the 

 adult animal in the situation of a thoracic 

 thymus, the very young animal, however, pre- 

 sents, in the same situation, true thymic struc- 

 ture, and the evidence of a real transformation 

 of the gland, not a mere replacement of 

 it by fat, seems more perfect than in the case 

 of the hibernators before referred to. It was 

 now a point of much interest to determine 

 the condition of the thymus in the fish-like 

 batrachian larva before its transformation had 

 occurred ; in none of these was any trace of 

 thymus detected, while in the youngest indi- 

 viduals in whom pulmonary respiration had 

 commenced the " commencement of the or- 

 gan" was always to be found. Mr. Simon 

 remarks ; " The essential step in reptile me- 

 tamorphosis is a higher developement of the 

 respiratory system : as a part of this pro- 



