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)i 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. 561, 562) 



This artery (A. carotis externa), by its sis^e 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 Ij^mph 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 maxillarj', 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 

 some 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. 



2 The cerebral arteries are very varialjle in arrangement, and the foregoing account is a brief 

 statement of the more usual disposition of the larger vessels. 



' Also termed the maxillo-musoular. 

 41 



