Basilar artery (Arlere mhso-ec'phalique, Chaussier).—This artery, larger than either of the vertebral arteries, is yet much less capa cious than the two vessels conjointly which unite to form it ; its length corresponds accurately to the longitudinal measurement of the pons varolii; it runs along the median depression of the pons, lodged between that body and the upper surface of the basilar process of the occipital bone, and preserved from pressure by the double concavity of the surfaces between which it is interposed.
At its commencement the basilar artery separates from each other the sixth nerves of the opposite sides ; the arachnoid membrane and the dura mater, with the transverse sinus of Haller, are interposed between the artery and the bone, whilst the pia mater alone intervenes between it and the pons. At the antero-superior edge of the pons the basilar artery usually terminates in four branches, two for the cerebrum and two for the cere bellum.
Branches of the vertebral artery. — I.
Branches to the prmvertebral muscles, which anastomose with the cervicalis arcendens and superficialis colli arteries.
2. Numerous small branches, which enter the spinal canal through the intervertebral foramina, and which are conducted by the nerves to the spinal cord, they join and reinforce the proper spinal arteries.
3. In the space between the atlas and occi pital bone, the vertebral artery sends several long branches down the neck, which are con cealed by the splenius and complexus muscles. Accompanied by smaller branches of the occi pital, these arteries keep up an important anastomosis on the back of the neck with the cervicalis profunda.
4, 5. The proper spinal arteries. These are two in number on each side, an anterior and a posterior branch from each vertebral trunk.
The posterior spinal arteries arise lower clown than the anterior, and pass downwards and backwards to reach the posterior surface of the medulla oblongata ; from this the artery of each side descends parallel to its fellow, to which it is connected by numerous transverse branches. Opposite the second lumbar vertebra, the posterior spinal arteries cease to exist as distinct trunks.
The anterior spinal arteries are given off from the vertebral near to its termination ; these arteries in descending approximate to each other, and at last unite opposite the lower edge of the medulla oblongata ; the single trunk thus formed (anterior median artery of the spinal cord) descends tortuously in front of the medulla spinalis, and, passing through the very centre of the fibres of the cauda cquina, reaches the lowest portion of the vertebral canal, when it anastomoses with branches of the sacral arteries.
The anterior and posterior spinal arteries are connected in every region of the spine with branches of arteries which enter the spinal canal through the " foramina of con junction ;" these reinforcing branches, as they may be termed, of the spinal arteries (derived from the cervicalis ascendens and vertebral in the neck, from the intercostal in the back, and from the lumbar and sacral arteries in the lower portion of the spinal column), cause these arteries which they join to pre serve a remarkable uniformity of size through out their entire course. Both the spinal arteries furnish small branches to the dura matey and to the spinal cord.
- Branches of the bstsilar artery. —1. The inferior (or posterior) cerebellar artery. This artery seldom arises in precisely the same manner at opposite sides of the same subject. It most frequently springs on one side from the vertebral, and on the other from the basilar trunk. Inclining outwards and backwards, in front of the pyramidal body, the vessel in question passes (according as its origin is from the basilar, or from the vertebral artery) before or behind the sixth nerve; it then runs through the filaments going to form the ninth nerve, and between the pneumogastric and spinal accessory divi sion of the eighth pair; it is ultimately dis tributed internally to the inferior vermiform process and sides of the median fissure (a branch or two may be traced into the choroid plexus of the fourth ventricle), and externally to the inferior surface anti circumference of the cerebellum, where it communicates with the superior artery of the cerebellum.
2. The superior (or anterior) cerebellar artery arises near the antero superior edge of the pons varolii ; it passes in a curved direction outwards and backwards, around the line of junction of the pons with the crus cerebri. It is at first parallel to the posterior artery of the cerebrum, but separated from it by the third nerve. The fourth nerve in its trajet forwards is strictly parallel to the artery as it runs backwards on the side of the pons varolii ; the nerve, however, is contained in a canal between the layers of the tentorium, whilst the artery proceeds beneath that par tition, and in contact with the upper surface of the cerebellum.