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the lateral wall of the nasal cavity. The anterior ethmoidal artery passes through the anterior ethmoidal canal with the anterior ethmoidal nerve, enters the anterior fossa of the skull, and crosses the lamina cribrosa of the ethmoid to the nasal slit, through which it reaches the nasal cavity where it descends, with the external branch of the nasal nerve, in a groove on the posterior surface of the nasal bone, and, finally, passes between the lateral cartilage and the Tower border of the nasal bone to the tip of the nose. It supplies branches to the membranes of the brain in the anterior cranial fossa as well as to the anterior ethmoidal cells, the frontal sinus, the anterior and upper part of the nasal muco-periosteum, and the skin on the dorsum of the nose.

(h) Palpebral branches, upper and lower, are given off near the termination of the ophthalmic. They are distributed to the upper and lower eyelids, and they anastomose with the lacrimal, supra-orbital, and infra-orbital arteries.

(2) The dorsal nasal terminal branch passes out of the orbit above the medial tarsal ligament. It pierces the palpebral fascia, and terminates on the side of the nose by anastomosing with the angular branch of the external maxillary artery.

(j) The frontal terminal branch pierces the palpebral fascia at the upper and medial part of the orbit, and ascends, with the supra-trochlear nerve, in the superficial fascia of the anterior and medial part of the scalp, anastomosing with its fellow of the opposite side and with the supra-orbital artery.

(6) The posterior communicating artery arises from the internal carotid near its termination. It runs backwards, below the optic tract and anterior to the pedunculus cerebri, and, passing above the oculomotor nerve, joins the posterior cerebral artery forming part of the circulus arteriosus (Willis). It gives branches to the optic chiasma, the optic tract, the pedunculus cerebri, the interpeduncular region, the internal capsule, and the optic thalamus. The posterior communicating artery varies much in size; it may be small on one or both sides, sometimes it is very large on one side ; occasionally it replaces the posterior cerebral artery, and it sometimes arises from the middle cerebral artery.

(7) The chorioidal is a small branch, which also arises near the termination of the internal carotid; it passes backwards and laterally, between the pedunculus cerebri and the uncus, to the lower and anterior part of the chorioidal fissure which it enters, and it terminates in the chorioidal plexus in the inferior cornu of the lateral ventricle. It supplies the optic tract, the pedunculus cerebri, the uncus, the posterior part of the internal capsule, the tail of the caudate nucleus, part of the lentiform nucleus, and the amygdaloid nucleus.

(8) Arteria Cerebri Anterior.—The anterior cerebral artery is the smaller of the two terminal branches of the internal carotid. It passes forwards and medially, above the optic chiasma and in front of the lamina terminalis, to the commencement of the longitudinal fissure; there it turns round the genu of the corpus callosum, and runs backwards to the parietal lobe of the brain. At the commencement of the longitudinal fissure it is closely connected with its fellow of the opposite side by a wide but short anterior communicating artery, and in the remainder of its course it is closely accompanied by its fellow artery of the opposite side.

Branches.-Branches of all the cerebral arteries are distributed both to the basal ganglionic masses of the brain and to the cerebral cortex ; they therefore form two distinct groups which do not communicate with one another—(a) central or basal ; (6) cortical.

The branches of the anterior cerebral include:

(a) Central or basal branches.—The antero-medial basal arteries, a small group of vessels, constitute the basal branches of the anterior cerebral artery ; they pass upwards into the base of the brain, in front of the optic chiasma, and supply the rostrum of the corpus callosum, the lamina terminalis, the head of the caudate nucleus, the anterior part of the lentiform nucleus and internal capsule, the columns of the fornix, the septum pellucidium, and the anterior commissure.

(6) Cortical branches. (61) Medial orbital, one or more small branches which supply the medial orbital convolution, the gyrus rectus, and the olfactory lobe.

(62) Anterior medial frontal, one or more branches which are distributed to the anterior and lower part of the medial surface of the superior frontal gyrus, and to the anterior portions of the superior and middle frontal gyri on the lateral surface of the hemisphere.

(13) An intermediate medial frontal is distributed to the posterior part of the medial lateral surfaces of the superior frontal gyrus and to the upper parts of the anterior and posterior central gyri.

(14) The posterior medial frontal runs backwards to the præcuneus. It supplies the corpus callosum, the præcuneus, and the upper part of the superior parietal lobule.

(9) Arteria Cerebri Media.—The middle cerebral artery is the larger of the two terminal branches, and the more direct continuation of the internal carotid artery. It passes laterally, in the stem of the lateral fissure (Sylvius), to the surface of the insula, and it divides, in the posterior part of the circular sulcus (Reil), into parieto-temporal and temporal terminal branches.

Branches.—(a) The central or basal, which constitute the antero-lateral basal arteries, are numerous and very variable in size. They arise at the base of the brain, in the region of the anterior perforated substance. Two sets, known as the medial and the lateral striate arteries, are distinguishable.

(a?) The medial striate arteries pass upwards through the two medial segments of the lentiform nucleus (globus pallidus) and the internal capsule to terminate in the caudate nucleus. They supply the anterior portions of the lentiform and caudate nuclei and of the internal capsule.

(a) The lateral striate arteries pass upwards through the lateral segment (putamen) of the lentiform nucleus, or between it and the external capsule, and they form two sets : an anterior, the lenticulo-striate, and a posterior, the lenticulo-optic ; both sets traverse the lentiform nucleus and the internal capsule, but the lenticulo-striute arteries terminate in the caudate nucleus, and the lenticulo-optic in the thalamus. One of the lenticulo-striate arteries, which passes in the first instance round the lateral side of the lentiform nucleus, and afterwards through its substance, is larger than its companions; it frequently ruptures, and is known as the “artery of cerebral hæmorrhage.”

(6) Cortical branches are given off as the middle cerebral artery passes over the surface of the insula at the bottom of the lateral fissure, as follows :

(81) The lateral orbital runs forwards and laterally, and is distributed to the lateral part of the orbital surface of the frontal lobe and to the inferior frontal gyrus.

(62) The inferior lateral frontal, which supplies the inferior and middle frontal gyri.

(53) The ascending frontal, which turns round the upper margin of the lateral fissure, and is distributed to the anterior central gyrus and to the posterior part of the middle

frontal gyrus.

(14) The ascending parietal branch emerges from the lateral fissure (Sylvius) and passes upwards along the posterior border of the posterior central gyrus, supplying that gyrus and the superior parietal lobule.

(65) The temporal branch passes out of the lateral fissure, and turns downwards to supply the superior and middle temporal gyri.

(56) The parieto-temporal branch continues backwards, in the direction of the main stem of the middle cerebral artery, and emerges from the posterior end of the lateral fissure ; it supplies the inferior parietal lobule, part of the lateral surface of the occipital lobe, and the posterior part of the temporal lobe.

ARTERIA VERTEBRALIS.

The vertebral artery (Figs. 757 and 761) is the first branch given off from the subclavian trunk; it arises from the upper and posterior part of the parent stem, opposite the interval between the anterior scalene and the longus colli muscles, and terminates at the lower border of the pons (Varolii) by uniting with its fellow of the opposite side to form the basilar artery.

Course and Relations.—The vertebral artery is divisible into four parts.

The first part runs upwards and backwards, between the scalenus anterior and the lateral border of the longus colli, to the foramen in the transverse process of the sixth cervical vertebra. It is surrounded by a plexus of sympathetic nerve fibres, is covered anteriorly by the vertebral and internal jugular veins, and it may be crossed anteriorly by the inferior thyreoid artery. On the left side the terminal part of the thoracic duct also passes anterior to it. The second part runs upwards through the foramina in the transverse processes of the upper six cervical vertebræ. As far as the second cervical vertebra its course is almost vertical; as it passes through the transverse process of the epistropheus, however, it is directed obliquely upwards and laterally to the atlas. It is surrounded by a plexus of sympathetic nerve fibres, and also by a plexus of veins. The artery lies anterior to the trunks of the cervical nerves, and medial to the intertransverse muscles. The third part

emerges from the foramen in the transverse process of the atlas, between the anterior division of the sub-occipital nerve medially and the rectus capitis lateralis laterally, and runs almost horizontally backwards and medially, round the lateral and posterior aspects of the corresponding superior articular process of the atlas. In this part of its course it enters the sub-occipital triangle, where it lies in the groove on the upper surface of the posterior arch of the atlas (sulcus arteriæ vertebralis). It is separated from the bone by the sub-occipital nerve, and is overlapped superficially by the adjacent borders of the superior and inferior oblique muscles. Finally, this

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part of the artery passes anterior to the oblique ligament of the atlas and enters the vertebral canal.

The fourth part pierces the spinal dura mater and runs upwards into the cranial cavity. It passes between the roots of the hypoglossal nerve, posteriorly, and the first dentation of the ligamentum denticulatum, anteriorly, pierces the arachnoid, and, gradually inclining to the front of the medulla oblongata, reaches the lower border of the pons, where it unites with its fellow of the opposite side to form the basilar artery

Branches.- From the first part. As a rule there are only a few small muscular twigs from this portion of the artery.

From the second part.—(1) Muscular branches which vary in number and size. They supply the deep muscles of the neck, and anastomose with the profunda cervicis, the ascending cervical, and the occipital arteries.

(2) Spinal branches pass from the medial side of the second part of the vertebral artery, through the intervertebral foramina, into the vertebral canal, where they give off twigs which pass along the roots of the spinal nerves to reinforce the anterior and posterior spinal arteries; they supply the bodies of the vertebrae and the intervertebral fibrocartilages, and they anastomose with corresponding arteries above and below.

From the third part.-(1) Muscular branches to the sub-occipital muscles.

(2) Anastomotic branches which unite with the descending branch (O.T. princeps cervicis) of the occipital and with the profunda cervicis artery.

From the fourth part.—(1) Meningeal.—One or two small branches given off before the vertebral artery pierces the dura mater. They ascend into the posterior fossa of the skull, where they anastomose with meningeal branches of the occipital and ascending pharyngeal arteries, and occasionally with branches of the middle meningeal artery.

(2) Posterior Spinal. --The posterior spinal branch springs most commonly from the posterior inferior cerebellar branch of the vertebral (Stopford, 1916), but occasionally it arises from the vertebral directly. It runs downwards upon the side of the medulla oblongata and the spinal medulla, either in front of or behind the posterior nerve-roots. It is a slender artery, which is continued to the lower part of the spinal medulla by means of reinforcements from the spinal branches of the vertebral and intercostal arteries. It gives off branches to the pia mater, which form more or less regular anastomoses on the medial and lateral sides of the posterior nerve-roots, and it ends by joining the anterior spinal artery.

(3) The anterior spinal branch arises near the termination of the vertebral. It runs obliquely downwards and medially, in front of the medulla oblongata, and unites with its fellow of the opposite side to form a single anterior spinal artery, which descends along the anterior median fissure of the spinal medulla, and is continued as a fine vessel along the filum terminale. The anterior spinal artery is reinforced as it descends by anastomosing twigs from the spinal branches of the vertebral, intercostal, and lumbar arteries. It gives off branches which pierce the pia mater and supply the spinal medulla, and it unites below with the posterior spinal arteries.

(4) The posterior inferior cerebellar is the largest branch of the vertebral artery. It arises a short distance below the pons and passes obliquely backwards round the medulla oblongata, at first between the fila of the hypoglossal nerve, and then between the fila of the accessory and vagus nerves, into the vallecula of the cerebellum, where it divides into lateral and medial terminal branches.

The trunk of the artery gives branches to the medulla oblongata and to the chorioid plexus of the fourth ventricle. Some of these branches supply the nuclei of the glossopharyngeal, the vagus, and the accessory nerves, the spino-thalamic, spino-cerebellar, rubrospinal, olivo-cerebellar tracts, and possibly also the vestibular root of the acoustic and the spinal root of the fifth nerve (Bury and Stopford). The medial terminal runs backwards between the inferior vermis and the hemisphere of the cerebellum; it supplies the former structure, and anastomoses with its fellow of the opposite side. The lateral branch passes to the lower surface of the hemisphere and anastomoses with the superior cerebellar artery.

Arteria Basilaris.— The basilar artery is formed by the junction of the two vertebral arteries; it commences at the lower border and terminates at the upper border of the pong (Varolii), bifurcating at its termination into the two posterior cerebral arteries.

Course and Relations. It runs upwards, in the median part of the cisterna pontis, in a shallow groove on the front of the pons, behind the sphenoidal section of the basi-cranial axis and between the two abducent nerves.

Branches.-(1) Pontine, a series of small arteries which pass across the front and round the sides of the pons, supplying the pons, the brachia pontis (O.T. middle peduncles of the cerebellum), and the roots of the trigeminal nerve.

(2) The internal auditory, a pair of long slender branches. Each internal auditory branch may spring either from the basilar or from the anterior inferior cerebellar artery of the same side (Stopford, 1916). It enters the corresponding internal acoustic meatus with the facial and acoustic nerves, and, after it has passed through the lamina cribrosa, it is distributed to the internal ear.

(3) The anterior inferior cerebellar, two branches which arise, one on each side, from the middle of the basilar artery. They pass backwards, on the anterior parts of the lower surfaces of the lateral lobes of the cerebellum, and anastomose with the posterior inferior cerebellar branches of the vertebral arteries.

(4) The superior cerebellar branches, two in number, arise near the termination of the basilar. Each passes laterally, at the upper border of the pons, directly below the oculo-motor nerve of the same side, and, after turning round the lateral side of the pedunculus cerebri, below the trochlear nerve, it reaches the upper surface of the cerebellum, where it divides into a medial and a lateral branch. The medial branch supplies the upper part of the vermis, and the anterior medullary velum. The lateral branch is distributed over the upper surface of the lateral lobe ; it anastomoses with the inferior cerebellar arteries.

(5) Arteriæ Cerebri Posteriores.— The posterior cerebral arteries (Figs. 762 and 764) are the two terminal branches of the basilar. They run backwards and upwards, between the peduncles of the cerebrum and the uncinate gyri and parallel to the superior cerebellar arteries, from which they are separated by the oculomotor and trochlear nerves. Each posterior cerebral artery is connected with the internal carotid by the posterior communicating artery; it gives branches to the inferior surface of the cerebrum, and is continued backwards, beneath the splenium of the corpus callosum, to the calcarine fissure, where it divides into calcarine and parieto-occipital branches, which pass to the lateral surface of the occipital lobe. It supplies the medial and tentorial surfaces of the occipital lobe and the posterior part of its lateral surface.

Branches.—(A) Central or basal.This group includes (al) A postero-medial set of small vessels which pass, on the medial side of the corresponding cerebral peduncle, to the posterior perforated substance. They supply the peduncle, the posterior part of the thalamus, the corpora mamillaria, and the walls of the third ventricle.

(ao) A postero-lateral set of small vessels, which pass round the lateral side of the peduncle. They supply the corpora quadrigemina, the brachia, the pineal body, the peduncle, the posterior part of the thalamus, and the corpora geniculata.

(as) A posterior chorioidal set of small branches which pass through the upper part of the chorioidal fissure ; they enter the posterior part of the tela chorioidea of the third ventricle, and end in the chorioid plexus, in the body of the lateral ventricle, and the upper part of its inferior cornu. They also supply the adjacent parts of the fornis.

(B) Cortical.-(01) The anterior temporal, frequently a single branch of variable size, but not uncommonly replaced by several small branches. It supplies the anterior parts of the uncus, the hippocampal gyrus, and the fusiform gyrus.

(62) The posterior temporal is a larger branch than the anterior. It supplies the posterior part of the hippocampal gyrus, part of the fusiform gyrus, and the lingual gyrus.

(13) The calcarine branch is the continuation of the posterior cerebral artery along the calcarine fissure, it is especially associated with the supply of the visual area of the cortex of the brain. It supplies the cuneus, the lingual gyrus, and the posterior part of the lateral surface of the occipital lobe.

(54) The parieto-occipital branch, smaller than the calcarine, passes along the corresponding fissure to the cuneus and præcuneus.

Circulus Arteriosus (Willis) (Fig. 764).—The cerebral arteries of opposite sides are intimately connected together at the base of the brain by anastomosing channels. Thus, the two anterior cerebral arteries are connected with one another by the anterior communicating artery, whilst the two posterior cerebrals are in continuity through the basilar artery from which they arise. There is also a free anastomosis on each side between the carotid system of cerebral arteries and the vertebral system by means of the posterior communicating arteries, which connect the internal carotid trunks and posterior cerebral arteries.

The vessels referred to form the so-called circulus arteriosus (O.T. circle of Willis) which is situated at the base of the brain, in the interpeduncular and chiasmatic subarachnoid cisterns. It encloses the following structures : the posterior perforated substance, the corpora mamillaria, the tuber cinereum, the infundibulum, and the optic chiasma. The “circle” is irregularly polygonal in outline, and is formed posteriorly by the termination of the basilar and by the two posterior cerebral arteries, postero-laterally by the posterior communicating arteries and the internal carotids, antero-laterally by the anterior cerebral arteries, and in front by the anterior communicating artery,

It is stated that this free anastomosis equalises the flow of blood to the various parts of the cerebrum, and provides for the continuation of a regular blood-supply if one or more of the main trunks should be obstructed.

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