In the neck the internal carotid artery usually gives no branch ; whilst within the carotid canal it sends a small offset to the tympanum, which anastomoses with the tympanic and stylo-mastoid arteries; within the cavernous sinus, some small branches, named arteries receptaculi, proceed from it to supply the walls of the sinus and the adjacent dura mater. One of these, distributed to the dura mater, is the anterior meningeal.


Opposite to the anterior clinoid process, the artery gives off the ophthalmic branch, and within the Sylvian fissure of the brain it divides into the anterior cerebral, the middle cerebral, and the posterior communicating arteries.

Ophthalmic artery. The ophthalmic artery, [a. ophthalmica] passes forwards from the internal carotid artery, where it lies by the anterior clinoid process, and enters the orbit by the foramen opticum, placed below and to the outer side of the optic nerve. It soon changes its direction, passing above and to the inner side of the nerve, to reach the inner wall of the orbit, along which it runs forwards, to terminate in branches which ramify on the side of the nose.

In its course the ophthalmic artery gives off numerous branches, which are destined to supply the eye and its appendages.

They are as follows :

The lachrymal artery

The lachrymal artery [a. lachrymalis] the first of the branches of the ophthalmic, is a long branch which arises from that vessel whilst on the outer side of the optic nerve. It passes forwards beneath the periosteum of the roof of the orbit, along the upper border of the external rectus muscle, and guided by it to the lachrymal gland, in which the greater number of its branches are distributed. Some of the branches pass onwards to the eyelids and conjunctiva, joining with other palpebral branches ; and one or two delicate vessels, malar branches, pierce the malar bone from the orbit to reach the temporal fossa, where they join branches from the deep temporal arteries. The lachrymal artery also sends branches through the sphenoidal fissure into the skull, which join with small offsets from the middle meningeal artery.

The central artery of the retina

The central artery of the retina, [a. centralis retinae] a very small vessel, pierces the sheath and substance of the optic nerve, and runs embedded within it until it reaches the retina, upon a vascular membrane in front of which it ramifies in minute branches. A very delicate vessel, demonstrable in the foetus, passes forwards through the vitreous humour, to reach the posterior surface of the capsule of the crystalline lens.

The supra-orbital branch

The supra-orbital branch, [a. supra-orbitalis] ascends to get above the muscles, and in its course forwards to the supra-orbital notch, accompanied by the frontal nerve, lies immediately beneath the roof of the orbit. The artery mounts towards the forehead, and distributes several branches, which communicate with those of the temporal artery, besides some which are distributed upon the eyelids.

The ciliary arteries

The ciliary arteries [a. ciliares] are divisible into three sets : viz., short, long, and anterior ciliary arteries. The short ciliary arteries vary from twelve to fifteen in number, and will be found to enclose the optic nerve as they pass forwards to reach the posterior aspect of the sclerotic coat, which they pierce, in order to enter the eyeball, about a line or two from the entrance of the optic nerve. The long ciliary arteries, two in number, also enter the back of the eye, and then pass forwards, one on each side of the middle of the eyeball, between the choroid membrane and the sclerotic, as far as the ciliary ligament, where they divide into branches. The anterior ciliary arteries are derived from some of the muscular branches ; they form a vascular circle around the fore part of the eyeball, and then pierce the sclerotic within a line or two of the margin of the cornea. All of these ciliary arteries anastomose together within the eyeball, where their distribution will be particularly described with the anatomy of the eye itself.

The muscular branches are subject to much variety in their course and distribution, like all muscular vessels: they supply the muscles of the orbit.

The ethmoidal branches

The ethmoidal branches [a. ethmoidalis post, et ant.] are two in number, a posterior and an anterior. The former passes through the posterior ethmoidal foramen in the inner wall of the orbit, and having given some small branches to the posterior ethmoidal cells, enters the skull, and, after supplying the adjacent dura mater, sends minute vessels, which descend through the foramina of the cribriform plate of the ethmoid bone to the nasal fossa?. The other, or anterior ethmoidal branch, passes with the nasal division of the ophthalmic nerve through the anterior fora- men, and having given branches to the anterior ethmoidal cells and frontal sinuses, reaches the interior of the skull, and like the preceding vessel, supplies the dura mater, and sends branches through the cribriform lamella to the nose.

The palpebral branches

The palpebral branches, superior and inferior, [a. palpebralis interna sup. et inf.] arise usually in common, but soon diverge as they pass forwards, one lying above, the other below, the tendon of the orbicularis muscle at the inner angle of the eye : they are distributed, as their names imply, to the upper and lower eyelids, branches being also sent to the caruncula laehrymalis and the lachrymal sac.

The nasal branch [a. dorsalis nasi] courses forwards above the tendon of the orbicularis muscle to the root of the nose, where it ramifies, maintaining a free communication with the nasal and the angular branches of the facial artery.

The frontal branch

The frontal branch, [a. frontalis] runs close to the preceding, but on reaching the margin of the orbit turns upwards on the forehead, where it anastomoses with the supra-orbital artery.

The terminal branches of the internal carotid artery, given off after it has pierced the layer of the dura mater forming the wall of the cavernous sinus, are those which supply the pia mater and the brain.

The anterior cerebral

The anterior cerebral (anterior cerebri sive corporis callosi), commences at the subdivision of the internal carotid as it reaches the inner end of the fissure of Sylvius. From this point it turns forwards towards the middle line to reach the longitudinal fissure between the anterior lobes of the cerebral hemispheres, across which fissure it is connected with the corresponding vessel of the opposite side by a branch, not more than two lines in length, named the anterior communicating, [a. communicans ant.]. The two anterior cerebral arteries, lying close together, in the next place turn round the anterior border of the corpus callosum, and, on reaching its upper surface, run from before backwards upon it, overlapped by the cerebral hemispheres, .and end by anastomosing with the posterior cerebral artery in the back part of the fissure. In this course numerous branches are given off by both arteries in various directions.

The middle cerebral artery

The middle cerebral artery, [a. media cerebri] , the largest branch of the internal carotid, inclines obliquely outwards, taking the course of the fissure of Sylvius, within which it divides into several branches, which supply the pia mater invest- ing the surfaces of the anterior and middle lobes of the brain, and join with the branches of both the anterior and posterior cerebral arteries. Some of its branches, as they ramify in the pia mater, turn forwards to and enter the brain at the ante- rior perforated spot, through which they reach the corpus striatum. One or two {choroid arteries) which sometimes arise directly from the internal carotid, will also be observed to enter the fissure between the middle lobe and the crus cerebri, to reach the descending cornu of the lateral ventricle, in which they are distributed to the choroid plexus.

The posterior communicating, [a. communicans post.], runs directly backwards, parallel with the corresponding artery of the opposite side, so that they enclose between them th£ infundibulum and the corpora albicantia ; they terminate in the posterior cerebral arteries, and thus form the sides of the circle of Willis.

Circle of Willis

Circle of Willis, [circulus arteriosus Willisii]. A remarkable anastomosis exists between the branches of the vertebral and internal carotid arteries within the cranium, by which the circulation in the brain maybe equalized, and any irregularity which might arise from the obliteration of one, or even two of the vessels, may speedily be remedied by a corresponding enlargement of the others. This anastomosis, which is known as the circle of Willis, results from a series of communications between the following branches. The anterior cerebral arteries are connected together in the longitudinal fissure by the anterior communicating artery. The internal carotids of each side, the trunks from which the anterior cerebral arteries arise, are united to the posterior cerebral arteries by the posterior communicating arteries, and the posterior cerebral arteries themselves arise behind from a single trunk — the basilar artery. Within, or opposite to the area of this vascular circle, will be observed the following parts of the encephalon, viz., — the commissure of the optic nerves, lamina cinerea, infundibulum and tuber cinereum, corpora albicantia, locus perforatus with part of the crus cerebri, and the origin of the third pair of nerves of each side.

Peculiarities of the branches of the internal carotid artery.

The lachrymal branch of the ophthalmic has been occasionally found, as already referred to, to be supplied by the anterior branches of the middle meningeal artery.

The deviations from the ordinary condition of the cerebral arteries mostly have reference to the mode in which the circle of Willis is completed. Thus the anterior communicating artery, which is usually very short and of good size, may be longer and smaller than usual ; and it has been found double either in the whole or in part of its length. Sometimes, but very rarely, this communicating branch is wanting, the two anterior cerebral branches of the internal carotid being then united at once into a single trunk (like the basilar artery behind), which after a certain course, again divides into the right and left anterior cerebral arteries or arteries of the corpus callosum (J. F. Meckel). — Another very rare condition of the anterior cerebral artery has been described by Arnold, in which one large anterior cerebral artery supplied the place of both as to its distribution, and was connected only by slender branches to the internal carotid of the opposite side.

The posterior communicating artery varies much in size, being sometimes very small, whilst, on the contrary, it is often found so large that the posterior cerebral artery may be said to spring from the internal carotid instead of from the basilar. The posterior communicating artery on one side is very frequently found larger than on the other; and it has occasionally been seen to be represented by two very slender vessels.

The internal carotid was in one instance observed to furnish a remarkable branch, which, after passing backwards through the basilar portion of the sphenoid bone, joined with the basilar artery, and formed the anterior part of that vessel. 




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