In the _Os Sphenoides_, or Wedge-like Bone, behind the Hole thro' which passeth the hindermost Fibre of the fifth Pair of Nerves lies another small Hole, almost round, that gives Entrance to a Branch of the _External Carotick_ Artery, which in entring, immediately adheres to the _Dura Mater_, and forms many Ramifications to overspread the whole Portion of this Membrane, which covers the sides, and the upper-part of the Brain. At the bottom and top of the lateral outward part of the Orbit of the Eye, above the acute Angle, for want of the _Os Sphenoides_, there is a Hole thro' which passeth an Artery, being a Twig of a Branch of the _Internal Carotick_, which is diffus'd in the Eye, and distributed to almost the whole Portion of the _Dura Mater_, that covers the fore-part of the Brain. The Vertebral Artery in entring into the Skull, furnisheth it on each side with a considerable Branch, which is dispers'd throughout the whole Portion of the _Dura Mater_ that covers the _Cerebellum_. {88} As for the Veins that accompany these Arteries, they almost all go out of the Skull thro' the same Holes where the other enters. There are four thick Arteries which convey to the Brain the Matter with which it is nourish'd, and that whereof the Spirits are form'd, _viz._ the two _Internal Caroticks_, and the two _Vertebrals._ The _Internal Carotick_ Arteries enter into the Skull thro a particular _Ductus_ made in the Temporal Bone, the Mouth thereof being of an Oval Figure and situated in the outward part of the _Basis_ of the Skull, before the Hole of the _Internal Jugular._ This _Ductus_ extends it self obliquely from the back-side forward, and after having made about three Hairs breadth in length, is terminated in the hinder-part of the _Os Sphenoides_. The Artery traverseth the whole winding Compass of this _Ductus_, which resembles the Figure of the _Roman_ Letter S, and at the Mouth of the same _Ductus_ runs under the _Dura Mater_ along the sides of the _Os Sphenoides_ to the _Anterior Apophyses Clinoides_, where it riseth up again, to perforate the _Dura Mater_, and to adhere to the Root of the Brain. These Vessels, in like manner, after their departure from the Bone of the Temples to the place where they pierce the _Dura Mater_, make a second Circuit in form of the _Roman_ Character S. At the place where these _Carotick_ Arteries penetrate the _Dura Mater_, they send forth a thick Branch, which enters into the Orbit of the Eye, by the lower-part of the Hole, thro' which the _Optick_ Nerve hath its Passage. {89} The _Vertebral_ Arteries proceeding from the Holes of the transverse _Apophyses_ of the first _Vertebra_, turn about in passing under the upper oblique _Apophyses_ of the seven _Vertebra's_: Afterward they perforate the _Dura Mater_, and running under the Marrow, enter into the Skull thro' the Occipital Hole; then inclining one toward another, they are re-united, and form only one single Trunk.