BRANCHES OF THE INTERNAL CAROTID. 411 



internal tarsal ligament. They send small branches to the conjunctiva, the caruncle 

 and the lachrymal sac, and then pass outwards between the orbicularis muscle and 

 the tarsus, to form arches, one near the free margin of each lid, with the palpebral 

 branches of the lachrymal artery. 



() The nasal branch courses forwards above the internal tarsal ligament to the 

 root of the nose, where it ramifies, maintaining a free communication with the nasal 

 and angular branches of the facial artery. 



(/) The frontal artery turns upwards round the inner end of the orbital arch, 

 and is distributed to the integument, muscles and periosteum of theinesial part of 

 the forehead, anastomosing with the supraorbital artery and with the corresponding 

 artery of the other side. 



Varieties. The ophthalmic artery sometimes (15 per cent.) crosses beneath, instead of 

 over, the optic nerve, and it may then in its intraorbital course lie entirely to the inner side 

 of the nerve. It has been seen occasionally entering the orbit through the sphenoidal fissure. 

 The lachrymal artery not unfrequently, and in rarer cases a large part or even the whole of 

 the ophthalmic itself, arises from the middle meningeal artery ; or the 'ophthalmic artery has 

 been seen to give off the middle meningeal. The lachrymal may also be reinforced by the 

 anterior deep temporal artery. All the above varieties may be explained as resulting from 

 the enlargement of normal anastomotic channels, the regular trunk being more or less 

 diminished or even obliterated. The mode of origin of the branches of the ophthalmic artery 

 is subject to great variation. There are often one or two additional ciliary trunks. The 

 supraorbital and posterior ethmoidal are both inconstant. The nasal branch is sometimes 

 large and supplies a deficiency of the facial artery. (F. Meyer, " Zur Anatomic der Orbital- 

 arterien," Morph..Jahrb., xii, 1886.) 



2. The posterior communicating artery passes backwards across the outer 

 angle of the interpeduncular space of the cerebrum, and above the third nerve, to 

 join the posterior cerebral branch of the basilar artery in front of the crus cerebri. 

 It varies much in size, and the arteries of the two sides are commonly unequal : the 

 right is more frequently the larger. The posterior communicating artery gives off 

 small twigs to the parts in the interpeduncular space, and one or two larger branches 

 which pass in between the crus cerebri and the corpus albicans to supply the inner 

 and fore part of the optic thalamus. 



3. The anterior choroid artery runs outwards and backwards in the cleft 

 between the temporal lobe of the cerebral hemisphere and the crus cerebri, to reach 

 the choroid plexus in the descending cornu of the lateral ventricle. It also supplies 

 branches to the posterior half of the optic tract, to the uncinate convolution, and to 

 the posterior limb of the internal capsule. l 



4. The anterior cerebral artery (iv), from the division of the internal carotid 

 in the vallecula Sylvii, runs nearly horizontally forwards and inwards above the optic 

 nerve to the beginning of the longitudinal fissure between the cerebral hemispheres 

 in front of the optic chiasma. As it enters this fissure it is connected with the 

 vessel of the opposite side by the anterior communicating artery, a small transverse 

 branch not more than two lines in length. The two anterior cerebral arteries, lying 

 close together, then turn round the genu of the corpus callosum, giving off branches 

 to the hemisphere, and terminate in the posterior internal frontal arteries (v. 

 infra). 



5. The middle cerebral artery (iii) forms both in size and direction the 

 continuation of the internal carotid (fig. 345). It passes upwards and outwards in 

 the fissure of Sylvius, and soon divides into its terminal branches which ramify over 

 the outer surface of the hemisphere. 



Varieties of the cerebral arteries. In some instances the anterior cerebral arteries are 

 united into a single trunk, like the basilar artery behind, and this again divides into right 



1 A. Kolisko, "Ueber die Beziehung der Arteria choroidea anterior zum hinteien Schenkel der 

 inneren Kapsel des Gehirnes," Wien, 1891. 



