The author deals in this paper with a tumor of the left carotid body in a 28 years old man. The tumor shows the microscopical characters of an endothelioma. The author brings out remarks on the genesis of the carotid body, ascribing to its cellular elements a mesoblastic and not a sympathogonial origin. He refers to the reflex of Hering which has its elective departure site from the first portion of the internal carotid artery, where is also localized the carotid body. On the other hand the writer endeavours to demonstrate what is the anatomical basis of that reflex.