THE EXTERNAL CAROTID ARTERY 641 
ward and joins the posterior cerebral branch of the basilar. It gives off the deep 
cerebral artery (A. cerebri profunda), which winds around the cerebral peduncle 
and is distributed chiefly to the mid-brain; it is often double. A smaller collateral 
branch is the anterior chorioid artery (A. chorioidea oralis), which passes along the 
optic tract and is distributed in the chorioid plexus of the lateral ventricle. 
The anterior cerebral artery (A. cerebri oralis) unites with the corresponding 
branch of the opposite artery above the optic chiasma. From this junction pro- 
ceeds the artery of the corpus callosum (A. corporis callosi), which turns around the 
genu of the corpus callosum, enters the great longitudinal fissure, divides into two 
branches, and is distributed to the medial aspect of the cerebral hemispheres. The 
anterior cerebral gives off the small internal ophthalmic artery (A. ophthalmica 
interna), which passes forward at first lateral to the optic nerve; then, crossing over 
the latter to its medial side, it joins a branch of the external ophthalmic. A small 
anterior meningeal branch (A. meningea oralis) of the anterior cerebral is distributed 
to the anterior part-of the dura, and assists in forming a network in the ethmoidal 
fossa (Rete ethmoidale), anastomosing with the ethmoidal branch of the external 
ophthalmic artery and the artery of the corpus callosum. 
The middle cerebral artery (A. cerebri media) passes outward in the fossa 
lateralis in front of the piriform lobe, reaches the lateral fissure, and divides into 
branches on the lateral surface of the hemisphere. 
The circulus arteriosus (Fig. 559)! is formed at the interpeduncular space of 
the base of the brain by the union of the anterior cerebral arteries in front, by the 
diverging posterior cerebral arteries behind, and is completed laterally by the junc- 
tion of the latter with the posterior communicating arteries and by the internal 
carotid. It is irregularly polygonal in outline, and surrounds the optic chiasma 
and pituitary body.” 
THE EXTERNAL CAROTID ARTERY (Figs. 56!, 562) 
This artery (A. carotis externa), by its size and direction, constitutes the con- 
tinuation of the common carotid. It passes forward on the lateral wall of the 
pharynx at the lower border of the guttural pouch, covered by the mandibular 
gland and the occipito-mandibularis, digastricus, and stylo-hyoideus muscles, and 
in relation to the parapharyngeal lymph glands. It then emerges between the stylo- 
hyoideus and the great cornu of the hyoid bone, ascends on the latter parallel with 
the posterior border of the lower jaw, and terminates about two inches (ca. 5 cm.) 
below the temporo-mandibular articulation by dividing into superficial temporal 
and internal maxillary branches. It is crossed deeply near its origin by the anterior 
laryngeal and pharyngeal branches of the vagus nerve. Just before its emergence 
its superficial face is crossed by the hypoglossal nerve, and the glosso-pharyngeal 
nerve passes over its medial surface at the ventral border of the great cornu. The 
chief collateral branches are the masseteric, external maxillary, and posterior 
auricular. It also furnishes variable branches to the mandibular and parotid 
glands, the guttural pouch, and the pharyngeal lymph glands, as well as twigs to 
seme adjacent muscles. 
1. The masseteric artery (A. masseterica)® is given off from the external 
carotid at its emergence from beneath the stylo-hyoideus. It passes downward 
and slightly forward under cover of the parotid gland and over the tendon of in- 
| sertion of the sterno-cephalicus to the posterior border of the mandible and appears 
on the masseter muscle, which it enters after a short course on its surface. It gives 
1 Also termed the circle of Willis. 
* The cerebral arteries are very variable in arrangement, and the foregoing account is a brief 
statement of the more usual disposition of the larger vessels. 
| ® Also termed the maxillo-muscular. 
