SUBCL AVIAN ARTERY. 313 



The Anterior cerebral artery passes forwards in the great longitudinal 

 fissure between the two hemispheres of the brain ; then curves back- 

 wards along the corpus callosum to its posterior extremity. 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 anterior 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 its origin it gives off the numerous small branches 

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

 stria turn. 



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 posterior cerebral 

 artery. 



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

 ternal carotid, near 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 distributed to the 

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



SUBCLAVIAN ARTERY. 



The Sultclavian artery, on the right side, arises from the arteria in- 

 nominata, 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 relations 

 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 

 perpendicularly to the inner border of that muscle. The second portion 

 curves outwards behind the scalenus anticus ; and the third portion 

 passes downwards and outwards beneath the clavicle, to the lower 

 border of the first rib, where it becomes the axillary artery. 



Relations, The first portion, on the right side, is in relation in front 

 with the internal jugular and subclavian vein at their point of 

 junction, and is crossed by the pneumogastric nerve, cardiac nerves, 



