The term [septum] is Latin, and means "wall", "division", or "partition". The plural form for [septum] is [septa]. Examples of use of this word are:

• Nasal septum: The osteocartilaginous partition between both sides of the nose
• Septum pellucidum: A transparent division between the lateral ventricles of the brain.
• Interatrial septum: The thin, muscular separation between the cardiac atria (see image, item "E")
• Interventricular septum: The muscular and membranous division between the ventricles of the heart (see image, item "D")

Image property of: CAA.Inc.. Photographer: D.M. Klein. 

The arachnoid mater, commonly referred to only as [arachnoid], is one of the three meninges that surround, cover, and protect the spinal cord and the brain. 

The first word is Greek, from [arachne] meaning "spider" and the suffix [-oid], meaning "similar to". The second word is Latin, from [mater], meaning "mother". The name arachnoid refers to the fact that in a fresh specimen the arachnoid mater looks quite like a spider's web. See the accompanying image. Click on the image for a larger version.

Embryologically, the arachnoid mater and the pia mater are considered to have a common origin, and are called the leptomeninges (thin meninges). These two membranes, internal to the dura mater, are very thin and separated by the subarachnoid space, which is filled with cerebrospinal fluid. 

Image property of: CAA.Inc.
Photographer: D.M. Klein

The word [thorax] is Greek and means "chest", "chest plate" or an area covered by a chest plate. The root term is [-thorac-].

The thorax is the region of the trunk found between the neck superiorly and the abdomen inferiorly. It is characterized by a bony cage (thoracic cage). It is bound anteriorly by the sternum, laterally by twelve pairs of ribs and corresponding cartilages, and posteriorly by the thoracic spine, composed by twelve thoracic vertebrae.

Its superior boundary is known as the thoracic inlet or superior thoracic aperture, and it an oblique plane formed by the superior aspect of the sternum, the first rib, and the first thoracic vertebra. The inferior boundary is more difficult to describe, as there are two boundaries. The first is the inferior thoracic aperture or outlet formed by the lower end of the sternum, the cartilage of the costal margin and the most inferior ribs. The second one is the boundary between thorax and abdomen. formed by the respiratory diaphragm.

The thoracic cavity is divided into three separate regions; two laterally situated pleural regions, each containing a lung, and a centrally situated mediastinum.  The mediastinum contains the heart and pericardium, the trachea and bronchi, thoracic duct, thoracic esophagus, etc.

Thorax - Anterior view. Click on the image for a larger version.

The anterior communicating artery is a single, unpaired artery found communicating both anterior cerebral arteries. The anterior communicating artery is one of the components of the arterial circle of Willis.

The anterior communicating artery is very short, usually between 2 to 3 mm in length, it is rarely absent, but it can be oblique or duplicated, or even triplicated. It is one of the sites for intracranial aneurysm. For more information on the anatomical variations of the anterior communicating artery click here.

For additional information:
"An Overview of Intracranial Aneurysms" Keedy, A Mcgill J Med. 2006 July; 9(2): 141–146 
Image modified from the original courtesy of Wikipedia.

The prefix [ipsi-] arises from the Latin [ipse] meaning "self" or "same". In medical terminology this prefix is used in the words "ipsilateral" and ipsilaterally" to denote an organ or structure in the "same side of the body" (left or right) as another structure that is being referred to.

A synonym for ipsilateral is [homolateral]; the antonym for the term is [contralateral].

The anterior cerebral arteries are one of the two paired terminal branches of the internal carotid artery. Each anterior cerebral artery supplies arterial blood to the brain beyond the arterial circle of Willis. The vascular territory of the anterior cerebral artery supplies the medial surfaces of the frontal and parietal cerebral lobes as well as the medial border of the same lobes.

The anterior cerebral artery gives off the orbitofrontal artery and then divides into two callosal arteries, the pericallosal artery and the callosomarginal artery. From these arteries arise all the other branches that form the vascular territory of the anterior cerebral artery.

There are many anatomical variations of the anterior cerebral artery, as described here.

Image modified from the original (in the public domain) by Sobotta (1945)