These are two small bodies situated, one on each side, between the
origins of the external and internal carotid arteries. Microscopically
they are divided into nodules or cell balls by connective tissue, and
these closely resemble the structure of the parathyroids, but are
without any thymic tissue. The blood-vessels in their interior are
extremely large and numerous. The modern view of their development is
that they are part of the sympathetic system, and the reaction of their
cells to chromium salts bears this out. (See Kohn, _Archiv f. mikr.
Anat._ lxx., 1907.)
In the Anura there is a rete or network into which the carotid artery
breaks up in the position of the carotid body, and this has an important
effect on the course of the circulation. It is probable, however, that
this structure has nothing to do with the carotid body of Mammalia.