DUCTLESS GLANDS. 363 



and the nuclei, spherical, relatively large, and containing from 

 one to three nucleoli. The free nuclei are also rounded and 

 contain several distinct nucleoli. These vesicles are easily 

 ruptured, when their contents exude in the form of an opa- 

 lescent fluid, sometimes called the thymic juice. 



Anatomists are somewhat divided in their opinions with 

 regard to the structure of the central cord and lobules. Some 

 adopt the view advanced by Sir Astley Cooper, 1 that the cord 

 has a central canal, connected with cavities in the lobules ; 8 

 while others believe that the cavities thus described are pro- 

 duced artificially, by the processes employed in anatomical 

 investigation. 3 The latter opinion is the latest, and is prob- 

 ably correct. 



The blood-vessels of the thymus are numerous, but their 

 calibre is small, and the gland is not very vascular. They 

 are derived chiefly from the internal mammary artery, a few 

 coming from the inferior thyroid, the superior diaphragmatic, 

 or the pericardial. They pass between the lobules, surround 

 and penetrate the vesicles, and form a capillary plexus in 

 their interior. The vesicles, in this respect, bear a certain 

 resemblance to the closed follicles of the intestine. The veins 

 are also numerous, but they do not follow the course of the 

 arteries. The principal vein emerges at about the centre of 

 the gland, posteriorly, and empties into the left brachio- 

 cephalic. Other small veins empty into the internal mam- 

 mary, the superior diaphragmatic, and the pericardial. A 

 few nervous filaments from the sympathetic system surround 

 the principal thymic artery, and penetrate the gland. Their 

 ultimate distribution is uncertain. The lymphatics are very 

 numerous. 4 



Inasmuch as the thymus is peculiar to early life, one of 



1 COOPER, Anatomy of the T/iymus Gland, London, 1832, p. 26, et seq. 



8 Cyclopaedia of Anatomy and Physiology, London, 1849-1852, vol. iv., Part 

 ii., p. 1087, Article, Thymus. 



* SAPPEY, Traite tfanatomie descriptive, Paris, 1857, tome iii., p. 456, and 

 LITTRE ET ROBIN, Dictionnaire de medecine, Paris, 1865, Article, Thymus. 



4 KOLLIKER, ffandbuch der Gewebelehre des Menschen, Leipzig, 1867, S. 485. 



