554 THE BLOOD-VASCULAR SYSTEM 



the superior meatus and upper spongy bones of the nose, and anastomose with the nasal branches 

 of the spheno-palatine artery (fig. 452). 



(7) The anterior ethmoidal artery [a. ethmoidalis anterior] (fig. 452), a larger branch 

 than the posterior ethmoidal, arises in front of the latter, passes medially between the superior 

 obhque and medial rectus, and, leaving the orbit through the anterior ethmoidal canal, in com- 

 pany with the anterior ethmoidal nerve, enters the cranial cavity. After running a short dis- 

 tance beneath the dura mater on the cribriform plate of the ethmoidal bone, it passes into the 

 nose through the horizontal sht-hke aperture by the side of the crista galh. Its terminal 

 branch passes along the groove on the under surface of the nasal bone, and emerges on the nose 

 between the bone and lateral cartilage, terminating in the skin of that organ. It gives off the 

 following branches in its course: — (i) Ethmoidal, to the anterior ethmoidal ceUs; (ii) anterior 

 meningeal artery, [a. meningea anterior] to the dura mater of the anterior fossa; (iii) nasal, 

 to the middle meatus and anterior part of the nose; (iv) frontal, to the frontal sinuses; (v) 

 cutaneous, or terminal, to the skin of the nose. 



(8) The medial palpebral arteries [aa. palpebrales mediales] arise either separately or by a 

 common trunk from the ophthalmic artery opposite the pulley for the superior oblique, just 

 as the latter vessel is about to divide into its terminal branches. They pass, one above and 

 one below, the medial palpebral hgament and then skirt along the upper and lower eyehds 

 respectively, near the free margin between the palpebral tarsi and the orbicularis muscle, 

 and form a superior and an inferior tarsal arch [arcus tarsus superior et inferior] by anastomosing 

 with the lateral palpebral branches of the lacrimal. The upper also anastomoses with the supra- 

 orbital artery and orbital branch of the temporal artery; the lower with the infraorbital, the 

 angular branch of the external maxillary (facial), and the transverse facial arteries. A branch 

 from the lower palpebral passes with the ductus nasolacrimalis as far as the inferior meatus. 

 Small twigs, the posterior conjunctival arteries [aa. conjunctivales posteriores], are also given 

 to the caruncula lacrimaUs and conjunctiva. 



(9) The frontal artery [a. frontahs], the upper of the terminal branches of the ophthalmic, 

 pierces the superior tarsus at the medial angle of the orbit, passes upward over the frontal bone 

 beneath the orbicularis oculi, supphes the structures in its neighbourhood, and anastomoses 

 with its fellow of the opposite side, with the supraorbital, and with the anterior division of the 

 superficial temporal artery. 



(10) The dorsal nasal [a. dorsahs nasi], the lower of the terminal branches of the ophthalmic, 

 leaves the orbit at the medial angle by perforating the tarsus above the medial palpebral liga- 

 ment. It then descends along the dorsum of the nose, beneath the integuments, and anasto- 

 moses with the angular and lateral nasal branches of the external maxillary (facial). It gives 

 off a lacrimal branch as it crosses the lacrimal sac, and a transverse nasal branch as it crosses 

 the root of the nose; the latter vessel anastomoses with its fellow of the opposite side. 



3. THE POSTERIOR COMMUNICATING ARTERY 



The posterior communicating artery [a. communicans posterior] (fig. 456) is 

 given off from the internal carotid just before the division of that vessel into the 

 anterior and middle cerebral arteries; occasionally it arises from the middle cere- 

 bral itself. 



It is as a rule a slender vessel which runs backward over the optic tract and pedunculus 

 cerebri along the side of the hippocampal gyrus to join the posterior cerebral. At times, how- 

 ever, it is of considerable size, and contributes chiefly to form the posterior cerebral, the portion 

 of the latter vessel between the basilar and posterior communicating being then as a rule reduced 

 to a mere rudiment. It gives off the following branches: — (a) the hippocampal, to the gyrus 

 of that name; and (6) the middle thalamic, to the optic thalamus. 



4. THE CHORIOID ARTERY 



The chorioid artery [a. chorioidea] is a small but constant vessel which arises 

 as a rule from the back part of the internal carotid j ust lateral to the origin of the 

 posterior communicating. 



It pa.sses backward on the optic tract and the pedunculus cerebri, at first lying parallel and 

 lateral to the posterior communicating artery. It then dips under the edge of the uncinate 

 gyrus and, entering the chorioid fissure at the lower end of the inferior cornu of the lateral 

 ventricle, ends in the chorioid plexus and supplies the hippocampus and fimbria. 



5. THE ANTERIOR CEREBRAL ARTERY 



The anterior cerebral artery [a. cerebri anterior] (figs. 450, 459), one of the 

 terminal hr!in(;he.s into \vlii(!h the internal carotid divides in the lateral fissure 

 (fi.'^sure of Sylvius), supplies a part of tiie cortex of the frontal and parietal lobes 

 of the brain and a small part of the basal ganglia. It passes at first anteriorly and 

 medially across the anterior perforated substance between the olfactory and optic 

 nerves to the longitudinal fissure where it approaches its fellow of the opposite side 



