Sphenoid part; Horizontal part; M1 segment

Sphenoid part; Horizontal part; M1 segment of middle cerebral artery: This sphenoidal segment is named due to its origin and loose lateral tracking of the sphenoid bone. Although known also as the horizontal segment, this may be misleading since the segment may descend, remain flat, or extend posteriorly the anterior (dorsad) in different individuals. The M1 segment perforates the brain with numerous anterolateral central (lateral lenticulostriate) arteries, which irrigate the basal ganglia.

The M1 segment is described by Gibo et al (Microsurgical anatomy of the middle cerebral artery - GiboH et al. - J Neurosurg. 1981 Feb;54(2):151-69.). as : 

The M1 segment began at the origin of the MCA and extended laterally within the depths of the Sylvian fissure. It lay an average of 9.4 mm (range 4.3 to 19.5 mm) posterior to the sphenoid ridge in the sphenoidal compartment of the Sylvian fissure. This segment ter- minated at the site of a 90° turn, the genu, located at the junction of the sphenoidal and operculoinsular compartments of the Sylvian fissure. The horizontal orientation of this segment, roughly parallel to the sphenoid ridge, has led to its being referred to as either the horizontal or the sphenoidal segment. The M1 segment was subdivided into a pre-bifurcation and post- bifurcation part. The pre-bifurcation segment was composed of a single main trunk that extended from the artery's origin to its bifurcation. The post-bifurcation trunks of the M1 segment ran in a nearly parallel course, diverging only minimally, prior to reaching the genu. This bifurcation occurred proximal to the genu in 86% of hemispheres. The small cortical branches arising from the main trunk proximal to the bifurcation are referred to as early branches.