344 



B. F. KINGSBURY 



vessels, gives to the 'descent' of the heart somewhat the character 

 of a growth funnel or eddy. This analogy is not without its 

 force in comprehending the growth transformations of the bran- 

 chial region, as will be seen subsequently. 



The vessels which have appeared in the process of growth 

 that has carried the second arch forward may even at this stage 

 be considered as the common carotid arteries, while the common 

 trunk for the first and second arches is the external carotid. At 

 this stage the common carotid artery springs rather directly 

 from the aortic trunk forming the bifurcation. Diagrams 1 

 and 2 in the accompaji3'ing schema B may illustrate the general 



^. 



1 r 



Schema B 



morphological relations of thyreoid and arches at this stage. 

 Continuance of lateral growth, together with relative caudal 

 displacement of the truncus, increases the length of the com- 

 mon carotid artery and causes it in its turn to move out along 

 the fourth aortic arch to its permanent position. 



The thyreoid gland in its expansion shares in the lateral 

 'upward' growth and median 'down sinking;' it then becomes 

 transformed into a U, already marking out the lateral lobes and 

 isthmus of the adult organ, the isthmic portion still being close 

 to the aortic bifurcation, while the lateral lobes are molded about 

 the common carotid arteries upon their medial and ventral sides, 

 as they have plastically followed them in their lengthening (figs. 

 2, 4, 6, 7, 15, 18). With the lateral growth of the thyreoid tfie 

 descent of the gland as a whole becomes arrested; it lags behind 

 the heart and" truncus, altering its relative position but little 

 in subsequent development. The growth of the ventral neck 



