THYMUS OF MAMMALIA. 



567 



439 



tissue of the mediastinum. At birth the bulk of the gland lies 

 behind the manubrium, descending to near the middle of the 

 sternum, and ascending upon the fore and lateral parts of the 

 trachea to the thyroid. By dissection the thymus can be sepa- 

 rated into two lateral portions, which are 

 naturally distinct at an earlier phase of de- 

 velopment ; each lateral part being a narrow 

 elongated body, folded upon itself, and further 

 resolvable into lobules and acini, like those ot 

 a true conglomerate gland : but all the acinal 

 cavities communicate with a central reservoir, 

 fig. 439, occupied by a milk-like solution ot 

 albuminoid or proteine principles. Formifac- 

 tion here produces ( corpuscles, very closely 

 resembling (in fact identical with) the nuclei 

 of glandular cells; '* but presenting more 

 numerous nucleoli: their form being for the 

 most part spherical. ' Mingled with these 

 I have found in the thymus of a Calf, as well 

 as in that of a young Guinea-pig, a few larger 

 corpuscles, about double the size of the former, 

 of spherical form, filled either with a granular 

 matter alone, or containing also a nucleus, or larger vesicular 

 body.' 2 



The thymus in Monotremes lies between the episternum and 

 the beginnings of the vessels from the aortic arch. In a Kan- 

 garoo from the pouch Simon found the thymus on the pericardium 

 with a medial lobe besides the two lateral ones. 3 In Rodents the 

 thymus consists of two long lobes extending from the base of the 

 heart, parallel with each other, forward, to the root of the neck. 

 Bodies extending from this position to the posterior mediastinum 

 and forward along the cervical vessels to near the mandible, but 

 consisting, according to Simon, of aggregates of fat-vesicles, 

 undergo periodical increase, in the Marmots, prior to hiber- 

 nation. In a Bat dissected in March, Dr. H. Jones could 

 not detect any certain homologue of a thymus ; but found 

 on each side of the root of the neck a yellowish lobulated mass 

 consisting of conical lobes defined by limitary membrane: the 

 lobes were hollow and ' filled by aggregations of celloid particles, 

 which were not manifestly nucleated, nor provided with an en- 

 velope, 4 but consisted of aggregations of oil-drops and molecules. 

 In the subjoined view, fig. 440, of a portion of this body, magni- 

 2 lb. 



Section of Thymus, showing 



the central reservoir. 



ccxvi". 



ccxiv" 



1093. 



ccxv" 



ccxiv'. p. 1096. 



