1322 THE SKIN, MAMMARY GLANDS AND DUCTLESS GLANDS 



nests are termed the concentric corpuscles of Hassall. The cortex is subdivided by secondary 

 connective-tissue septa extending in from the septa between the lobules. 



The arteries of the thymus are somewhat varied in their origin, usually derived from the 

 internal mammary and inferior thyreoid of each side; branches are sometimes received from the 

 innominate, subclavian, and superior thyreoid arteries. They reach the gland in various places 

 and spreading out in the capsule pass with the trabeculae through the gland to form a plexus 

 around each small lobule. From this capillaries pass through the cortex to the medulla. 

 The veins issue from the thj-mus in various places and are seen as numerous branches on its 

 surface. The efferent vessels drain into various veins, mostly into the left innominate vein, 

 also smaller branches into the internal mammary and inferior thyreoid veins. 



The lymphatics arise around the small lobules and pass through the interlobular septa to 

 the surface from which thej^ are drained into small lymph nodes near the cephaUc extremity, 

 into glands ventrally between the thymus and the sternum, and into other glands dorsally be- 

 tween the thj'mus and the pericardium. 



The nerves of the thymus are very minute. They are derived from the cervical sympa- 

 thetic and from the vagus and reach the thymus for the most part along with the blood- 

 vessels which they accompany through the septa. 



Fig. 1076. — Thymus in a Child of Two Years. 



Thyreoid cartilage 



Central portion of crico-thyre«id 



membrane 

 Crico-thyreoideus 



First ring of trachea 

 Thyreoid gland 



Seventh ring of trachea ^^ " " i~ 



Right carotid artery 



Right subclavian artery 





Right innominate vein 



Thymus 



Vena cava superior ^ — 



Arch of aorta 



Ligament connecting thyreoid aad 

 thymus gland 



-Left carotid artery 



— Left subclavian artery 



Lobe of thymus passing 

 behind vein 



-Left innominate vei» 



.Arch of aorta 



V 



"^ -^ y 



Development. — The thymus ari.ses from the endodermal portion of the third pharyngeal 

 pouch on each side, as a thickening due to an increase in the epithehal cells, followed by the 

 production of a diverticulum. At about the sixth week the connections of the pouches with the 

 branchial clefts are cut off but a strand of tissue may persist to represent the stalk. These 

 thick-wallod cylinders become solid cords, elongate so as to extend caudally into the thorax, 

 and enlarge by a scries of secondary IjudcHngs. The glands of the two sides come into contact 

 and become intimately a.s.sociatcd. The cephalic portion, as a rule, later atrophies and disap- 

 pears. Occasionally a small part of it remains near the thyreoid cut off from the rest of the 

 gland as an accessory thynnis. From the fourth pharyngeal pouch rarely a thymus bud may 

 be developed which produces in the adult also an accessory thymus. The epithelial character 

 of the cells remains plainly evident for a lime, then the characteristic differentiation into 

 lymphoid structure, cortex and medulla appears. The reticulum and concentric corpuscles 

 arc undoubtediv of epithelial origin; but the thymus lymphocytes are considered by Hammar 

 and others as leucocytes which have migrated to the thymus, while they are regardedj by 



