48 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



branches of the inferior thyroid arteries. These are derived from 

 the lateral thyroid buds. In structure they are made up of 

 reticulating columns of cells, with vessels arranged between the 

 columns, thus resembling in structure the carotid body, and 

 probably also in nature and origin the medullary part of the 

 supra-renal. Their presence is essential to the function of the 

 thyroid body. 



Carotid Bodies. — The carotid body occurs at the inner side of 

 the fork, between the internal and external carotid arteries. The 

 commencement of the internal carotid represents the artery of the 

 3rd arch ; that of the external carotid, the ventral aortic trunk. 

 The body therefore appears to be developed in the wall of the 

 pharynx at the ventral end of the 2nd cleft. It receives a large 

 supply of nerves from the superior cervical ganglion, and it con- 

 tains a rich network of vessels. Swale- Vincent regards it as 

 similar in nature and origin to the coccygeal body and medulla of 

 the supra-renal (see p. 259). 



