i8oo 



HUMAN ANATOMY. 



Fir, 



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I ^20. 



Capsule 



Cortex 



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

 are traceable along the arteries and connective-tissue septa, and end chiefly in the 

 walls of the blood-vessels. Bovero has described terminal filaments which pass from 



the interlobular plexuses into 

 the medulla. 



Development. — The 

 thymus proper originates from 

 a paired anlage (Fig. 152 1) 

 which appears as an epithelial 

 outgrowth from the A'entral 

 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- 

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

 puscles of Hassall represent derivatives of the primary epithelial elements. For a 

 time the two originally distinct anlages develop independently ; later they come into 

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



Section of thymus body 



Hassall 



showing; details of cortical and medullary 

 substance. • 200. 



Fig. 



1521. 



Fec^nstructions of developintj thyroid, thymus, and parathyroid bodies in embryos of 14 mm. (A) and of 26 mm. 

 {B). /, true thyroid; It, so-called lateral thyroid; i_y, thymus; /', Z^, superior and inferior parathyroids; re, vena 

 cava superior; a, aorta. ( Tourneux and Verdun.) 



derivation of which is indicated by the connecti\'e 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 



