Posterior cerebral artery

PCA divided four segments.
P1: extend from its origin from the basilar artery to the junction with the P com.
P2 From the Pcom to the posterior aspect of the midbrain.
P3 From the posterior aspect of the midbrain to the calcarine fissure.
P4 The terminal branches of the PCA distal to the anterior limit of the calcarine fissure.


Branches
PCA branches divided into three categories:
1. Perforating branches, to the brainstem and thalamus, arise from the P1 and P2 segments.
2. Ventricular branches, originate mostly from the P2 segment.
3. Cortical branches, arise from the P2, P3, and P4 segments.

P1 segment
Aka precommunicating, mesencephalic or horizontal segment, lies immediately superior to the oculomotor and trochlear nerves. The average length is 6.6 mm.

Branches: 
Perforators
1. Direct perforating branches (aka posterior thalamoperforating arteries) from the P1 segment pass directly into the brainstem. These are termed the posterior thalamoperforators to distinguish them from the anterior thalamoperforators, which arise from the P-comm artery.
2. Circumflex arteries. The circumflex arteries (aka peduncular, mesencephalic, or tegmental thalamoperforating arteries) arise from the P1 and P2 segments and encircle the midbrain. Subdivided into short and long circumflex arteries.
Posteromedial choroidal artery. This vessel usually arises from the P2 segment, may arises from the P1 segment.
Meningeal branch, aka artery of Davidoff and Schecter, a small branch from P1 segment to supply a midline strip of the inferior surface of the tentorium may be enlarged by pathological processes.

Variations. 
Asymmetry of P1 segments common, being present in ~50% of angiograms. When a fetal PCA is present, the ipsilateral P1 is typically hypoplastic or absent.
Persistent carotid–vertebrobasilar anastomoses, the PCA may be supplied by branches from the carotid system.
True anomalies of the P1 segment are uncommon, includs duplication, fenestration  and a bilateral shared origin of the PCA and SCA.
Congenital absence of the P1 is rare.
Artery of Percheron, a single prominent perforating branch that supplies both the thalami and mesencephalic mid brain.

P2 segment
Aka ambient segment is relatively long, ~ 50 mm in length, travels around the lateral aspect of the midbrain within the ambient cistern, parallel and inferior to the basal vein of Rosenthal. Other adjacent structures are the trochlear nerve, the free edge of the tentorium, and the superior cerebellar artery.
Branches
Perforators
1. Thalamogeniculate arteries, supply the posterior half of the lateral thalamus, the posterior limb of the internal capsule.
2. Peduncular perforating arteries pass directly into the cerebral peduncle and supply multiple structures within the
3. Circumflex arteries.
Posteromedial choroidal artery, often arises from the P2 segment, may arise from P1 segment.
Hippocampal artery.
Inferior temporal arteries, anterior, middle and posterior temporal artery.
Parieto-occipital artery.
Calcarine artery.
Splenial artery.

P3 segment

Aka quadrigeminal segment, extends in a medial and posterior direction ~ 40mm in length.
The PCA often divides into its two terminal branches, the calcarine and parieto-occipital arteries between

Branches
Parieto-occipital artery.

P4 segment
Includes two main terminal branches of the PCA, the calcarine artery and parieto-occipital artery,

Branches
Calcarine artery, travels posteriorly and medially within the calcarine fissure to reach the occipital pole.
Splenial artery, aka posterior pericallosal artery arises from the parieto-occipital artery in 62% of cases, but may arise from the calcarine (12%), travels superiorly around the splenium of the corpus callosum to anastomoses with the pericallosal artery.

Reference: Handbook of Cerebrovascular Disease and Neurointerventional Technique.  Mark R. Harrigan, John P. Deveikis and Agnieszka Anna Ardelt.