46 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



from the surrounding mesoblast, the thymus becomes an adenoid 

 structure, the epithelial parts becoming compressed into the 

 corpuscles of Hassall. The surrounding mesoblast supplies its 

 connective tissue stroma and capsule. The lateral lobes come 

 together under the ventral aortae, and in the retrogression of the 

 heart are carried backwards to lie in the superior mediastinum. 

 The pointed upper extremity of each lateral lobe can be traced up- 

 wards in the fully developed foetus, under the lateral lobes of the 

 thyroid towards the thyro-hyoid membrane. These apical strands 

 represent the stalk of the thymic buds. In its growth back- 

 wards it crosses dorsal to the lateral thyroid buds which arise 

 from the 4th cleft. 



The thymus reaches its fullest growth in early childhood (3rd 

 or 4th year), and continues large as long as the body is in a 

 state of active growth. It shrivels up when maturity is reached, 

 and only a remnant is left as a rule, less remaining in men than 

 in woman. It receives its blood supply from the 4th aortic 

 arches through the internal mammary. In manner of origin it 

 resembles the tonsil ; indeed it may be regarded as a buried tonsil. 



The Thyroid. — At a very early period (4th week), while 

 the buccal and pharyngeal parts of the tongue are appearing as 

 elevations on the floor of the primitive pharynx, the hypoblast in 

 the mesial part of the furrow between those two parts of the 

 tongue thickens. The bud thus formed grows downwards and 

 backwards and soon bifurcates (Fig. 34). The bifurcated 

 extremity, after redivision to form a network of acini, becomes 

 the isthmus or median lobe of the thyroid. The stalk of 

 the bud becomes the thyro-glossal duct, the lingual opening 

 of the duct remaining as the foramen caecum. It seems 

 probable that this part of the thyroid, at least, was originally a 

 gland which poured its secretion into the mouth. The connective 

 tissue and vessels of the thyroid are derived from the surrounding 

 mesoblast ; only the glandular elements arise from the hypoblast 

 of the pharynx. 



Thyro-glossal Duct.— In the great majority of subjects the 

 thyro-glossal duct completely disappears ; the foramen caecum 

 marks one extremity, while a pyramid of thyroid tissue prolonging 

 the isthmus towards the hyoid bone often marks the other 

 extremity. The pyramid of the isthmus may carry on it a 



