THYMUS GLAND. 



1097 



neck to the vicinity of the lower jaw, spread- 

 ing itself out above the clavicles on each side 

 of the neck, and even passing behind the 

 clavicles and pectoral muscles into the axil- 

 lary spaces." Mr. Simon in his dissections 

 of the same family finds the masses described 

 by Tiedemann, as existing in the anterior and 

 posterior mediastina, but not those in the 

 neck, and states from microscopic examination 

 the remarkable fact, that they all consisted 

 of aggregations of fat vesicles. I do not ga- 

 ther clearly from his description whether these 

 fat cells were surrounded by a limitary mem- 

 brane, preserving the form of the thymic 

 cavities, in all the situations where the fat 

 masses are mentioned as existing ; i. e. whe- 

 ther all this fatty tissue was actually deve- 

 loped from the thymus, or a mere accumula- 

 tion in the ordinary way : nor does Mr. Simon 

 state whether the specimens he examined were 

 in a state of torpor at the time of their death 

 or not. I confess I should wish to have fur- 

 ther evidence concerning the accumulations of 

 fat in the marmot's thymus. I have never seen 

 a really nucleated fat cell, nor have I ever ob- 

 served any approach to such a development 

 of nuclei. Tiedemann's description indicates 

 so strongly a great difference between the 

 conditions of the masses in question at the 

 commencement of winter and in the summer, 

 that it seems to me quite necessary that a 

 comparative examination of their minute 

 structure at both periods should be under- 

 taken before the question of their relation to 

 the thymus can be decided. 



In Edentata the thymus is principally 

 thoracic, the Sloth and Armadillo present 

 small cervical prolongations connected to the 

 larger mass by very slender strips. 



In Monotremata a thymus is found resting 

 on the origin of the great vessels, and scarcely 

 extending into the neck. 



Among Pachydermata there are marked 

 differences in the form and development of 

 the thymus. A foetal Elephant presented a 

 flat mass resting on the upper two-thirds of 

 the pericardium, and sending upwards a short 

 prolongation from its right lobe. In a Peccary 

 the cervical portions were of great length, 

 not only reaching to the angle of the jaw, but 

 folded down again beside the trachea. So- 

 lipeds have a thymus either entirely thoracic 

 or reaching but a little way into the neck. 



Among Ruminants, the Calf is usually re- 

 ferred to as exhibiting a thymus, which may 

 serve as a type for the family. The gland 

 presents cervical cornua which are highly 

 developed, reach within the angle of the jaw 

 on each side, and form large complicated 

 masses up to the base of the cranium ; below, 

 these prolongations are narrowed, and united 

 in close juxtaposition to form an isthmus, 

 which passes behind the first bone of the 

 sternum, inclines to the left side, and expands 

 into another considerable mass of glandular 

 substance situated on the upper part of the 

 pericardium and covered by the left pleura. 

 In the Reindeer the submaxillary enlargements 

 are absent, and the tracheal are of compen- 



sative size. In the Fallow deer, again, the cer- 

 vical portions are more developed, extending 

 up to the cranium, but not reaching below 

 quite to the pericardium. 



A thymus has been found in several Ce- 

 tacea. In the fcetal Dolphin there are two 

 large median portions, pericardiac and tracheal, 

 with deep seated lateral connecting cornua : 

 in the Mysticete whale there were two peri- 

 cardiac lobes, from the right one of which a 

 prolongation extended backwards across the 

 arch of the aorta, between the art. innomin. 

 and left carotid till it reached the trachea, 

 and ascended a small distance in front of that 

 tube to terminate in two little cornua : the 

 left lobe was of much smaller size. 



Aves. In the class of birds no organ unequi- 

 vocally possessing the characteristic structure 

 of a thymus had until lately been shown to 

 exist. Meckel, indeed, described in diving birds 

 an organ which he supposed to be peculiar to 

 them, and which he considered to be the 

 thymus. The microscope, however, in Mr. 

 Simon's hand has demonstrated this organ to 

 be merely a mass of fat, and that, too, even 

 in the youngest specimens ; the same observer 

 has succeeded in discovering a real thymus, 

 which corresponds closely in structure with 

 that of mammalia at an early period of deve- 

 lopment. It is found in the neck of very 

 young birds, lying along the outer side of the 

 superficial cervical vessels, extending from 

 within a line or two of the base of the cra- 

 nium to just above the inlet of the thorax. It 

 appears as a semi-transparent ampullated tube 

 closed at both extremities, and rather broader 

 at its upper than at its lower part. Under the 

 microscope it is seen to have walls formed by 

 a distinct limitary membrane, and contents 

 consisting of nuclei. The period of its dis- 

 appearance varies somewhat, being earliest in 

 the tribes of powerful and active flight, but 

 in all occurring very much sooner than in 

 mammalia. 



This discovery of Mr. Simon's appeared to 

 me of so much interest, both as regards the 

 existence of the organ in birds, and the fur- 

 ther elucidation of its structure which might 

 be expected from the examination of the or- 

 gan in a lower grade of organized beings, that 

 I was anxious to repeat the observation. 



The result was entirely confirmatory of Mr. 

 Simon's description. In a chick about one week 

 old I found on each side of the neck a tube, 

 which appeared ampullated to the naked eye, 

 and which under the microscope exhibited a 

 large central cavity with numerous irregular 

 and slight lateral bulgings (Jig. 732.). It 

 was filled with imperfect nuclei, and with 

 granular globules, which last were more nu- 

 merous than usual ; there was also a quantity 

 of oily matter in the form of minute drops or 

 molecules. It was remarkable that on the 

 right side the tube was divided into a number 

 of separate portions, which seemed to have 

 become quite isolated from each other ; 

 this, I conceive, to have been the commence- 

 ment of a process of degeneration, an idea 

 which seems also favoured by the existence of 



