The Hole thro' which runs the _Auditory_ Nerve, that makes the seventh Pair, is in the middle of the hinder-part of the _Os Petrosum_, that looks toward the _Cerebellum_: This Hole being very large, is the Entrance of a _Ductus_ that is hollow'd in the _Os Petrosum_, and which sinking obliquely from the fore-part backward, for the depth of about two Hairs breadth, forms as it were the bottom of a Sack, the lowermost part whereof is terminated partly by the _Basis_ of the _Cochlea_, and partly by a Portion of the Mouth of the _Vestibulum_. At the bottom of this _Ductus_ are many Holes, but the most considerable is that of the upper-part, thro' which passeth a Portion of the Auditory Nerve. This is also the Entrance of another _Ductus_ made in the _Os Petrosum_, which is open'd between the _Apophysis Mastoides_ and _Styloides_: These other Holes afford a Passage to the Branches of the soft Portion of the same Auditory Nerve. {86} Below this _Ductus_ there is a remarkable Hole form'd by the meeting of two hollow Cuts the larger whereof is in the Occipital Bone and the other in the lower-part of the _Apophysis Petrosi_: From the middle of the upper-part of this Hole issueth forth a small Prominence or bony Point, whereto is join'd an Appendix of the _Dura Mater_, which divides the Hole into two parts; so that thro' the foremost Orifice passeth the Nerve of the eighth Pair, and that which is call'd the _Spinal_ Nerve. We shall have occasion hereafter to shew the Use of the hinder Orifice. Near the great Hole of the Occipital Bone from whence proceeds the _Medulla Oblongata_, we may observe a Hole almost round and oblong thro' which passeth the Nerve of the ninth Pair. This Hole is entirely situated in the Occipital Bone, and making a little Way in the Bone passeth obliquely from the back-part forward. In the inside of the Skull this Hole is sometimes double, but its two Entrances are re-united in the outward-part of the Skull; and the two Branches that form the Origine of this Nerve and which pass thro' these two Holes, are likewise re-united at their Departure, These are the Passages of the nine Pairs of Nerves that proceed from the _Medulla Oblongata_, and it remains only to show that Paths thro' which the Intercostal Nerve goes forth, as also that of the tenth Pair. The Intercostal runs out of the Skull thro' the _Ductus_ that gives Entrance to the Internal _Carotick_ Artery. As for the tenth Pair, in regard that it ariseth from the Marrow which is enclos'd between the Occipital {87} Bone and the first _Vertebra_, it goes forth thro' the Hole of the _Dura Mater_, where the Vertebral Artery enters. To know well the Holes thro' which the Vessels that belong to the inner-part of the Head enter, and issue forth, it is requisite to distinguish them into those which are distributed to the _Dura Mater_, and those that are appointed for the Brain. The Vessels of the _Dura Mater_, are Branches of the _Carotick_ or Vertebral Arteries.