Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/196

156 is no such hole. The use of it, when found, is for the transmission of a vein from the external teguments to the lateral sinus: But, in some subjects, a branch of the occipital artery passes through this hole, to serve the back-part of the dura mater.

The internal surface of the ossa temporum is unequal; the upper circular edge of the squamous part having numerous small ridges and furrows for its conjunction with the parietal bones; and the rest of it is irregularly marked with the convolutions of the middle part of the brain, and with furrows made by the branches of the arteries of the dura mater.

From the under part of this internal surface, a larger transverse, hard, craggy protuberance runs horizontally inwards and forwards, with a sharp edge above, and two flat sides, one facing obliquely forwards and outwards, and the other as much backwards and inwards. To the ridge between these two sides, the large lateral process of the dura mater is fixed.

Sometimes a small bone, akin to the sesamoid, is found between the small end of this petrous process and the sphenoid bone. Towards the back-part of the inside of the os temporum, a large deep fossa is conspicuous, where the lateral sinus lies; and frequently on the top of the petrous ridge, a furrow may be observed, where a small sinus is situated.

The internal proper foramina of each of these bones are, first, the internal meatus auditorius in the posterior plain side of the petrous process. This hole soon divides into two; one of which is the beginning of the aquæduct of Fallopius: the other ends in several very small canals, that allow a passage to the branches of the portio mollis of the seventh pair of nerves, into the vestibule and cochlea. Through it also an artery is sent, to be distributed to the organ of hearing.The second hole, which is on the anterior plain side of the craggy process, gives passage to a reflected branch of the second branch of the fifth pair of nerves, which joins the portio dura of the auditory nerve, while it is in the aquæduct, small branches of blood-vessels accompanying the nerves, or passing through smaller holes near this one.—The passage of the cutaneous vein into the lateral sinus, or of a branch of the occipital artery, is seen about the middle of the large fossa for that sinus; and the orifice of the canal of the carotid artery is evident at the under part of the point of the petrous process.

The upper round part of the squamous bones is thin, but equal; while the low petrous part is thick and strong, but irregular and unequal, having the distinction of tables and diploe confounded, with several cavities, processes, and bones within its substance, which are parts of the organ of hearing. See the description of the bones, muscles, &c. of the ear, in part VI.

The temporal bones are joined above to the parietal bones by the squamous sutures, and their posterior additamenta: Before, to the sphenoid bone by the suture of that name; to the cheek-bones by the zygomatic sutures: Behind, to the occipital bone, by the lambdoid suture and its additamenta; and they are articulated with the lower jaw in the manner which shall be described when this bone is examined.

, so called from its situation, is convex on the outside, and concave internally. Its figure is an irregular square, or rather rhomboid; of which the angle above is generally a little rounded; the two lateral angles are more finished, but obtuse; and the lower one is stretched forward in form of a wedge, and thence is called by some the cuneiform process.

The external surface is convex, except at the cuneiform apophyse, where it is flatted. At the base of this triangular process, on each side of the great hole, but more advanced forwards than the middle of it, the large oblong protuberances, named the condyles, appear, to serve for the articulation of this bone with the first vertebra of the neck. The smooth surface of each of these condyloid processes is longest from behind forwards, where, by their oblique situation, they come much nearer to each other than they are at their back-part. Their inner sides are lower than the external, by which they are prevented from sliding to either side out of the cavities of the first vertebra.-Round their root a small depression and spongy roughness is observable, where the ligaments for surrounding and securing their articulations adhere.—Though the motion of the head is performed on the condyles, yet the centre of gravity of that globe does not fall between them, but is a good way further forward; from which mechanism it is evident, that the muscles which pull the head back must be in a constant state of contraction: which is stronger than the natural contraction of the proper flexors, else the head would always fall forwards, as it does when a man is asleep, or labours under a palsy, as well as in infants, where the weight of the head far exceeds the proportional strength of these muscles. All round the great foramen the edges are unequal, for the firmer adhesion of the strong circular ligament which goes thence to the first vertebra.-One end of each lateral or moderator ligament of the head, is fixed to a rough surface at the fore-part of each condyle, and the perpendicular one is connected to a rough part of the edge of the great hole between the two condyles.

On the inner surface of the os occipitis we see two ridges; one standing perpendicular, the other running horizontally across the first. The upper part of the perpendicular limb of the cross, to which the falx is fixed, is hollowed in the middle, or often on one side, for the reception of the superior longitudinal sinus, and the lower part of it has the small or third process of the dura mater fastened to it, and is sometimes hollowed by the occipital sinus. Each side of the horizontal limb is made hollow by the lateral sinuses inclosed in the transverse process of the dura mater; the fossa in the right side being generally a continuation of the one made by the longitudinal sinus in the perpendicular limb, and therefore is larger than the left one. Round the middle of the cross there are four large depressions separated by its limbs; the two upper ones being formed by the back part of the brain, and the two lower ones by the cerebellum.Farther forward than the last mentioned pressions,