THE INTERNAL CAROTID ARTERY) (carotis in terna seu cerebralis, Scemm. c&dwale anarieure, Chaussier .)This artery is larger than the external carotid in the fcetus, but in the adult is only equal in size to that vessel, aud sometimes even smaller. At its origin it takes a curve outwards so as to get external to the commencement of the ex ternal carotid; it then mounts upwards and forwards in front of the three superior cervical vertebrw, and making a few contortions along the side of the pharynx, enters the foramen caroticum of the temporal bone, traversing the carotid canal of that bone internal to the ca vernous sinus, perforates the dura mater internal to the anterior clinoid process of the sphenoid bone, where it divides into two large branches, the anterior and middle cerebral.
The internal carotid artery has the following relations from its origin to the place where it enters the foramen caroticum : anteriorly it has the external carotid and its branches in contact with it at its origin, also the hypoglossal or lin gual nerve, and as it passes under the digastric muscle it also slips beneath the following parts which lie between it and the external carotid, the styloid process, with the muscles attached to it, part of the parotid gland, the glosso pharyngeal and inferior pharyngeal nerves.
Posteriorly it lies on the rectus capitis anti cus major, having the par vagum and superior laryngeal nerve behind it, and higher up the trunk of the hypo-glossal nerve coming from between it and the internal jugular vein.
The internal jugular vein bounds it externally at first, but passes to its posterior side above where it gets to the internal side of the root of the styloid process. Internally the carotid ar tery lies on the side of the pharynx to which it is more closely applied towards its upper part, lying on the stylo-glossus and the outer surface of the superior constrictor muscles, which with some cellular membrane and a venous plexus separate it from the tonsil, external and poste rior to which it lies, at the distance of from six to eight lines in the natural state of the parts; but when that gland is enlarged in consequence either of acute inflammation or chronic disease, the distance between it and the artery is dimi nished so much as to expose the latter to some risk of being wounded in opening abscesses in the tonsil, an occurrence of which the records of experience are not without examples. In
this stage of its course the internal carotid seldom gives any branches; occasionally, how ever, the inferior pharyngeal or the occipital arises from it. IIaving entered the carotid canal, the artery ascends vertically, then turns forwards and inwards, and passing out of the canal opposite the posterior clinoid process, it takes a second turn upwards, then forwards along the side of the sella turcica, between the layers of the dura mater which include the ca vernous sinus, between which latter and the bone the artery is situate. At the anterior extremity of the side of the sella turcica it makes a third turn upwards under the anterior clinoid process, and passing backwards and a little in wards it perforates the dura mater between the in ternal side of this process and the commissure of the optic nerves. The only vessels which it gives from its entrance into the foramen caroticum to the place where it perforates the dum mater are one or two small bmnches which perforate the petrous portion of the temporal bone, and pass to the cavity of the tympanum, and as it lies beside the cavernous sinus, two or three little twigs to the dura mater, pituitary gland, body of the sphenoid bone, and to the third, fourth, fifth, and sixth pairs of nerves which lie ex ternal to it and in contact with the outer or inner wall of the cavernous sinus.
The ophthalmic artery arises from the an terior side of the carotid while that vessel is passing into the dura mater, by the side of the anterior clinoid process; it enters the foramen opticum at first external and inferior to the optic nerve, over which it mounts obliquely towards its internal side, passing between it and the superior rectus muscle of the eye ; it then directs its course along the superior and internal part of the orbit between the obliquus superior and rectus internus, towards the inner canthus of the eye where it terminates. Before entering the orbit it gives off a few small twigs to the dura mater and cavernous sinus, and within the orbit it furnishes the following branches:— 1. the lachrymal ; 2. the arteria centralis retinue; 3. the supra-orbital; 4. the ciliary; 5. the muscular ; 6. the etlimoidal; 7. the palpe bm1; 8. the frontal ; and 9. the nasal.