I Sou 



HUMAN ANATOMY. 



The nerves are small and come from the sympathetic and the vagus. They 

 are trar-al.lc along the arteries and connective-tissue septa, and end chiefly in the 



.all, .f A. 



FIG. 1520. 



.Capsule 



Cortex 



Bovero has described terminal 



the 

 the medulla. 



Development. -- The 

 thymus proper originates from 

 a paired anlage (Fig. 1521) 

 which appears as an epithelial 

 outgrowth from the ventral 

 wall of the third pharyngeal 

 pouch. From this results a 

 long cylindrical mass of closely 

 packed epithelial cells which 

 grows downward and en- 

 closes a narrow lumen. The 

 lower end of this mass in- 

 creases in size by the formation 

 of solid acinous outgrowths 

 resembling those of an im- 

 mature tubo-alveolar gland. 

 Coincident with the downward 

 extension of the organ, the 

 upper cylindrical portion grad- 

 ually assumes the alveolar con- 

 dition until the entire thymus 

 acquires a lobulated character. 

 During these changes histo- 

 logical alterations take place, 

 the epithelial masses becoming 

 invaded by ingrowing lym- 

 phoid tissue and blood-vessels 

 and broken up into irregular islands. The latter become smaller and less conspicu- 

 ki the lymphoid character of the thymus becomes more predominant. The cor- 

 pusdespf llassall represent derivatives of the primary epithelial elements. For a 

 time the tu<. originally distinct anlages develop independently ; later they come into 

 contact in the." mid-line, and form the single irregular organ the bilateral 



"':'. fT'"-'\ .".?:. ' 

 *'*, ' 



<>! | mscles of Hassall 

 within medulla 



Section of thymus body, showing details of cortical and medullary 



FIG. 1521. 



V. 

 (ft). 



ti ; thvroi.l, thymus. and parathyroid bodies in embryos of 14 mm. (A) and of 26 mm. 

 r.inl; /.', scu.illr.l l.itmd thyroid; /v, thymus; /',/', superior and inferior parathyroids; i>c, vena 

 aorta. (flNMMIU '"</ / 'ttdun.) 



of which is indicated by tho connective tissue separating the right and left 

 divisions. The upper ends of the latter are often continued as far as the thyroid 

 as lateral processes. Subsequent to the second year regression sets in, and the 



