_What is the _Pericranium_?_

It is a Membrane that lies under the thick hairy Skin of the Head, and immediately covers the Skull.

_What are the _Meninges_?_

They are two Membranes that enclose the Substance or Marrow of the Brain.

_What is a Suture?_ {81}

It is a kind of thick Seam or St.i.tch, that serves to unite the Bones of the Skull.

_How many sorts of Sutures are there?_

There are two sorts, _viz._ the true, and the false or b.a.s.t.a.r.d.

_What are the true Sutures?_

They are three in number, namely the _Sagittal_, the _Coronal_, and the _Lambdoidal._

_What is the disposition or situation of the true Sutures?_

The _Sagittal_ is streight, beginning in the middle of the Fore-head, and sometimes at the root of the Nose, and being terminated behind, at the joining of the two Branches of the _Lambdoidal Suture_.

The Coronal appears in form of a Crown, pa.s.sing to the middle of the Head, and descending thro' the Temples, to finish its Circ.u.mference in the Root of the Nose.

The _Lambdoidal_ Suture is made like an open Pair of Compa.s.ses, the Legs whereof are extended toward the Shoulders; and the b.u.t.ton is in the top of the Head backward.

_What are the b.a.s.t.a.r.d Sutures?_

They are those that are call'd _Squamous_ or scaly.

_What is the disposition of natural situation of these false Sutures?_

They are plac'd at the two sides of the Head, and make a Semi-Circle of the bigness of the Ears, round the same Ears.

_What difference is there between the true and spurious Sutures._

The true Sutures are made in form of the Teeth of a Saw, which enter one into the other; and the false or b.a.s.t.a.r.d ones are those that resemble the Scales of Fishes, which {82} are join'd together by pa.s.sing one over the other.

_What is the Use of the Sutures?_

The Ancients were of Opinion, that they were made to hinder the Fracture of one Skull-Bone from pa.s.sing thro' the whole Head; but there is more reason to believe that they have the three following Uses, that is to say, 1. To promote the transpiration of the Brain. 2. To give Pa.s.sage to the Vessels that go to the _Diploe_. 3. To retain the _Meninges_, and to support the Ma.s.s of the Brain, which is cover'd by them.

_What are the Names of the Bones that compose the Skull?_

The Bone of the fore-part of the Head is call'd _Sinciput_, or the Fore-head-Bone, as also the _Frontal_ or _Coronal_ Bone. The Bone of the hinder-part, enclos'd within the _Lambdoidal_ Suture, is term'd the _Occipital._ The two Bones that form the upper-part, and are distinguish'd by the Sagittal Suture, bear the Name of _Parietals_, one being on the Right side, and the other on the Left. And those behind the Ears are call'd _Temporal_, _Squamosa_, or _Petrosa_. These also are distinguish'd into the Right and Left Temporals, and are join'd to the bottom of the Parietal by a b.a.s.t.a.r.d squamous Suture.

_What is most remarkable in the thickness of the Skull-Bones?_

The _Diploe_, which is nothing else but a _Plexus_ or Contexture of small Vessels, that nourish the Bones, and in the middle of their thickness make the distinction of the first and second Tablature of the Bones; whence it sometimes {83} happens that an exfoliative Trepan, or Semi-Trepan, is sufficient, when the first of these two Tables is only broken, the other remaining entire.

_Is the Brain which is preserv'd in the Skull all of one Piece, or one equal Ma.s.s?_

No, it is distinguish'd by the means of the _Meninges_ into the Brain it self, and the _Cerebellum_ or little Brain; the Brain, properly so called, takes up almost the whole Cavity of the Skull, and the _Cerebellum_ is lodg'd altogether in the hinder-part, where it const.i.tutes only one entire Body; whereas the former is divided into the Right and Left Parts by the _Meninges_, which cut it even to the bottom; whence these Foldings are call'd _Falx_; i. e. a _Scythe_ or _Sickle_.

_What is chiefly remarkable in the Substance of the Brain?_

The Ventricles or Cavities which are found therein, together with the great Number of Veins, Arteries, Lymphatic Vessels, and Nerves, that carry Sense to all the Parts of the Body, and Spirits for their Motion.

_An exact Historical Account of all the Holes of the Skull, and the Vessels that pa.s.s thro' them._

To attain to an exact Knowledge of all the Holes with which the inside of the _Basis_ of the Skull is perforated, they are to be consider'd either with respect to the Nerves, or to the Sanguinary Vessels. {84}

There are nine Pairs of Nerves that arise from the _Medulla Oblongata_, and go forth out of the Skull through many Holes hereafter nam'd.

The first Pair is that of the _Olfactory_ Nerves, appropriated to the Sense of Smelling, which are divided below the _Os Cribiforme_, or Sieve-like Bone, into divers Threads, that pa.s.sing into the Nose through many Holes with which this Bone is pierc'd, are distributed to the inner Tunick of the Nose.

The second Pair is that of the _Optick_ or Visual Nerves, that pa.s.s into the Orbits of the Eyes, thro' certain peculiar Holes made in the _Os Sphenoides_, or Wedge-like Bone, immediately above the _Anterior Apophysis Clinoides_.

In the Portion of the _Os Sphenoides_, that makes the _Basis_ of the Orbit, lies a Fissure about seven or eight Hairs breadth long, which is to be observ'd chiefly at the bottom, that is to say, below the Hole, thro' which the Optick Nerve pa.s.seth; where it is almost round, and larger than at the top, where it is terminated in a very long and acute Angle.

There are many Pairs of Nerves that enter into the Orbit thro' this Fissure, _viz._ 1. The third Pair, call'd the _Motorii Oculorum_. 2. The fourth Pair, nam'd _Pathetici_, by Dr. _Willis_. And 3. The whole sixth Pair. Besides these three Pairs, which go entire thro' this Cleft, there is also a Pa.s.sage for the upper Branch of the foremost Fibre of the fifth Pair, which the same renowned Physician calls the _Ophthalmick_ Branch.

Beyond the lower-part of the said Fissure, toward the hinder-part of the Head, is to be seen {85} in the _Os Sphenoides_ on each side, a Hole that doth not penetrate the _Basis_ of the Skull, but makes a kind of _Ductus_, about an Hair's breadth long, which is open'd behind the Orbit on the top of the s.p.a.ce between the _Apophysis Pterygoides_, and the third Bone of the Jaw; thro' this _Ductus_ runs the lower Branch of the foremost Fibre of the fifth Pair.

About the length of two Hairs breadth beyond these _Ductus's_, we may also discover in the _Os Sphenoides_, or Wedge-like Bone, two Holes of an Oblong and almost Oval Figure, which are plac'd in the hindermost sides of that of the _Os Sphenoides_, and gives pa.s.sage to the hindermost Fibre of the fifth Pair.

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 pa.s.seth 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 Pa.s.sage 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 pa.s.seth 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 pa.s.seth the Nerve of the ninth Pair. This Hole is entirely situated in the Occipital Bone, and making a little Way in the Bone pa.s.seth 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 pa.s.s thro' these two Holes, are likewise re-united at their Departure, These are the Pa.s.sages 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.

In the _Os Sphenoides_, or Wedge-like Bone, behind the Hole thro' which pa.s.seth 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 pa.s.seth 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 Compa.s.s 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 Pa.s.sage. {89}

The _Vertebral_ Arteries proceeding from the Holes of the transverse _Apophyses_ of the first _Vertebra_, turn about in pa.s.sing 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.