568 THE DUCTLESS GLANDS ENDOCRIN GLANDS 



organ. Pappenheimer regards the specific cells of the thymus as epi- 

 thelial in character, simulating, but not identical with, lymphocytes. 



Blood Supply. The larger arteries of the thymus are distributed 

 within the interlobular connective tissue. They supply branches to the 

 lobule which penetrate to the medulla, where they form a plexus of 

 sinusoidal capillaries with elongated meshes, and also distribute radi- 

 ating capillaries to the cortical portion of the lobule. These sinusoidal 

 vessels are highly characteristic of the medulla of the thymus lobule. 

 They unite to form venous radicals of considerable caliber, which leave 

 the lobule to join the interlobular veins in the loose connective tissue 

 between the lobules. 



Lymph vessels are of frequent occurrence in the iuterlobular con- 

 nective tissue, and their branches occasionally penetrate to the medulla 

 of the lobule, but there is nothing corresponding to the lymph sinuses 

 of lymph nodes. Small non-medullated nerve trunks are also found 

 in the interlobular connective tissue, but seem to be chiefly distributed 

 to the walls of the larger blood-vessels. 



Development and Function of the Thymus. The thymus arises as 



a pair of tubular outgrowths from the ventral aspect of the third pharyn- 

 geal pouch, which subsequently become solid and fuse to form a median, 

 flat, bi-lobed organ lying in the root of the neck and the upper portion of 

 the thorax. According to certain investigators (Stohr, et al.) the lymphoid 

 cells of the definitive organ are derived from the entodermal cells of the 

 initial anlages. The recent work of Maximow (Arch. mikr. Anat, 74, 3, 

 1909), however, seems to demonstrate that these cells are true lymphocytes 

 and that they have their origin in the surrounding mesenchyme, from the 

 cells of which they differentiate, and from which location they migrate into 

 the thymus anlages a conclusion confirmed by the still more recent work 

 of Badertscher (Amer. Jour. Anat., 17, 4, 1915). The original lymphocytes 

 are said to be of the large variety ; these give rise through proliferation with- 

 in the thymus to the smaller types. Among the invading leukocytes are also 

 a small number of polymorphs. The original epithelial anlages continue to 

 grow for a time, and gradually become differentiated into the definitive 

 reticulum of the thymus. Hassall's corpuscles are believed to represent non- 

 reticular remnants of the original entodermal anlages. According to Ham- 

 mar, and Bell, these concentric corpuscles are derived from hypertrophic 

 reticular cells. They first appear early in fetal life, and continue to form 

 and increase in size during infancy. They are interpreted by some as thy- 

 mic elements of internal secretion. The thymus is commonly regarded as a 

 hemopoietic organ, but its activity is limited to the formation of lympho- 

 cytes and possibly a small number of granulocytes, Beard (1900) views it 



