SUBCLAVIAN ARTERY. 281 



in the substance of the iris, and form a second circle around the 

 pupil. The anterior ciliary are branches of the muscular arteries ; 

 they terminate in the great arterial circle of the iris. 



The Centralis retina artery pierces the optic nerve obliquely, 

 and passes forwards in the centre of its cylinder to the retina, 

 where it divides into branches, which ramify in the inner layer of 

 that membrane. It supplies the retina, hyaloid membrane, and 

 zonula ciliaris ; arid, by means of a branch sent forwards through 

 the centre of the vitreous humour in a tubular sheath of the hyaloid 

 membrane, the capsule of the lens. 



The Anterior cerebral artery passes forwards in the great longi- 

 tudinal fissure between the two hemispheres of the brain; then 

 curves backwards along the corpus callosum to its posterior extre- 

 mity. It gives branches to the olfactory and optic nerves, to the 

 under surface of the anterior lobes, the third ventricle, the corpus 

 callosum, and the inner surface of the hemispheres. The two ante- 

 rior cerebral arteries are connected soon after their origin by a 

 short anastomosing trunk, the anterior communicating. 



The Middle cerebral artery, larger than the preceding, passes out- 

 wards along the fissure of Sylvius, and divides into three principal 

 branches, which supply the anterior and middle lobes, and the island 

 of Reil. _ Near to its origin it gives off the numerous small branches 

 which enter the substantia perforata, to be distributed to the corpus 

 striatum. 



The Posterior Communicating artery, very variable in size, some- 

 times double, and sometimes altogether absent, passes backwards 

 and inosculates with the posterior cerebral, a branch of the basilar 

 artery. Occasionally it is so large as to take the place of the pos- 

 terior cerebral artery. 



The Choroidean is a small branch which is given off from the 

 internal carotid, near to the origin of the posterior communicating 

 artery, and passes beneath the edge of the middle lobe of the brain 

 to enter the descending cornu of the lateral ventricle. It is distri- 

 buted to the choroid plexus, and to the walls of the middle cornu. 



SUBCLAVIAN ARTERY. 



The Subclavian artery, on the right side, arises from the arteria 

 innominata, opposite the sterno-clavicular articulation, and on the 

 left, from the arch of the aorta. The right is consequently shorter 

 than the left, and is situated nearer to the anterior wall of the chest ; 

 it is also somewhat greater in diameter, from being a branch of a 

 branch, in place of a division from the main trunk. 



The course of the subclavian artery is divisible, for the sake of 

 precision and surgical observation, into three portions. The first 

 portions of the right and left arteries differ in their course and rela- 

 tions in correspondence with their dissimilarity of origin. The other 

 two portions are precisely alike on both sides. 



The first portion, on the right side, ascends obliquely outwards to 

 the inner border of the scalenus anticus. On the left side it ascends 



36 



