144 MICROSCOPIC ANATOMY OF THE ORGANS. 



the periphery of concentrically arranged semilunar cells, while 

 the centre usually contains nuclear and cell detritus. 



According to Afanassiew, the Hassal's corpuscles arise from 

 vascular epithelium, which in the involution of the organ pro- 

 liferates until it fills the lumen. 



It seems that the thymus at first takes part in the formation 

 of red and white blood-corpuscles. Nucleated red corpuscles 

 and evidences of mitotic division are met with frequently. 



FIG. 102. 



m^^$^^"~'- 



5^^'J4^A '^ y> ^ "" vv!f <fi> 



Nuclei of 

 epithelial 

 cells 



Two Hassal's corpuscles from a section through the thymus of a child six months 



old. X 470. 



The arteries which extend into the interior of the lobule 

 from the connective-tissue septa break up at the inner boundary 

 of the cortical substance to form a fine capillary network. 

 Some of the branches of this supply the cortex and empty into 

 the veins at the periphery ; while other branches run to the 

 medullary substance, from which veins collect to carry the blood 

 back to the connective-tissue septa. 



Fine nerve plexuses have been observed in the septa and in 

 the medullary substance. 



5. THYROID GLAND. 



The thyroid is an alveolar gland possessing no duct. It 

 consists of a connective-tissue framework in the form of an 

 outer capsule, with strands of connective tissue dividing the 

 gland into lobules, and finer septa separating the individual 

 alveoli which make up the secretory portion. 



