658 
Fig. 4 shows the further changes in the development of the carotid 
arteries. 
The dorsal connection between the three aortic arches, 3, 4 and 
6, is unusually large as compared with other specimens of the same 
age. The fourth arch (a. 4) of this side is reduced to thread-like 
calibre. The sixth arch (a. 6) gives rise to the pulmonary artery 
(pl. a.) running in the same horizontal plane with the subclavian, while 
from the third arch spring three vessels — the definitive subclavian 
(scl. a.), slightly deflected in this figure in order to expose the pul- 
monary artery which arises in the same horizontal plane, the posterior 
remnant of the ventral carotid (car. v.), and the vertebral (ver. a.). _ 
(The vertebral is generally a branch of the common carotid.) 
Fig. 4. Camera sketch of dissection of the injected arterial system of the head 
and neck region of the left side of a chick embryo incubated seven and one-half days, 
showing the degeneration of the middle part of the ventral carotid. X about 16 dia- 
meters. 
The common carotid (¢. e.), as it passes forward dorsal to the 
pharynx, bifurcates in the region above the base of the lower and 
upper jaw into its external (car. ex.) and internal carotid (car. m.) 
branches. The internal carotid has some branches which are distributed 
superficially but its main stem passes to the brain, spreading out over 
its parts. The auricular artery (art. au.) springing from the internal 
carotid, has now three main branches, the mandibular (md), feeding 
the superficial muscles of the lower jaw, the supraorbitalis (sw. orb.) 
and infra-orbitalis (inf. orb.). The external carotid (car. ex) is now 
well established and exhibits its three main adult branches: the occipital 
(ocp. a.) which supplies the base of the brain, and the two branches 
