Part 168

The =anterior= or =occipital branch= (Ramus occipitalis) passes through the alar (antero-external) foramen of the atlas and supplies the muscles and skin of the poll, anastomosing with the deep cervical artery and its fellow of the opposite side. In the alar furrow it gives off the =cerebrospinal artery=, which enters the spinal canal through the intervertebral (antero-internal) foramen of the atlas, perforates the dura mater, and divides into cerebral and spinal branches. The =cerebral branch= unites with that of the opposite side to form the =basilar artery=, and the =spinal branch= similarly forms by union with its fellow the =middle spinal artery=.

The =basilar artery= (A. basilaris cerebri) passes forward in the median groove on the ventral surface of the medulla and pons and divides into the two posterior cerebral arteries. The collateral branches of the basilar are:

1. Medullary branches (Rami medullares), ten or twelve in number, distributed to the medulla oblongata.

2. Posterior cerebellar arteries (Aa. cerebelli caudales) which pass outward around the medulla behind the pons to the cerebellum, to which they are distributed after giving twigs to the medulla and pons.

3. The small auditory artery (A. auditiva interna) accompanies the eighth nerve to the internal ear. It often arises from the posterior cerebellar.

4. Anterior cerebellar arteries (Aa. cerebelli nasales). These are very variable in number and origin. There are often two or three on either side and they frequently arise from the posterior cerebrals. They pass outward in front of the pons and supply the anterior part of the cerebellum.

The =posterior cerebral arteries= (Aa. communicantes caudales) diverge at an acute angle and join the posterior communicating branches of the internal carotid arteries on the inferior surface of the cerebral peduncles. They are connected by a transverse branch and by a network of fine twigs which form often a rete mirabile.

THE INTERNAL CAROTID ARTERY (Figs. 434, 436, 437)

This artery (A. carotis interna) is somewhat smaller than the occipital. It usually arises just behind that artery, crosses its deep face, and runs upward and forward on the guttural pouch to the foramen lacerum. It is closely related to the vagus nerve and the superior cervical ganglion of the sympathetic nerve, fibers from which accompany it. It is crossed externally by the ninth and twelfth cranial nerves and the pharyngeal branch of the vagus. It passes through the inferior petrosal sinus and enters the cavernous sinus, within which it forms an =S=-shaped curve. It is connected with the opposite artery by a transverse branch (A. intercarotica), which lies behind the pituitary body in the intercavernous sinus. A branch (A. caroticobasilaris) sometimes connects it with the basilar artery. It then perforates the dura mater, gives off the posterior communicating branch, and passes forward and divides at the side of the optic chiasma into anterior and middle cerebral arteries.

The =posterior communicating artery= (A. communicans posterior) turns backward and joins the posterior cerebral. It gives off the =deep cerebral artery= (A. cerebri profunda) which winds around the cerebral peduncle and is distributed chiefly to the mid-brain. A smaller collateral branch is the anterior choroid artery (A. chorioidea nasalis) which passes along the optic tract and is distributed in the choroid plexus of the lateral ventricle.

The =anterior cerebral artery= (A. cerebri anterior) unites with the corresponding branch of the opposite artery above the optic chiasma. From this junction proceeds the =artery of the corpus callosum= (A. corporis callosi), which turns around the genu of the corpus callosum, enters the great longitudinal fissure, divides into two branches, and is distributed to the inner aspect of the cerebral hemispheres. A small =anterior meningeal= branch (A. meningea nasalis) of the anterior cerebral is distributed to the anterior part of the dura, and assists in forming a network in the ethmoidal fossa (Rete ethmoidale), anastomosing with the ethmoidal branch of the ophthalmic artery.

The =middle cerebral artery= (A. cerebri media) passes outward in the lateral fissure (of Sylvius) and divides into branches on the outer surface of the hemisphere.

The =arterial circle of Willis= (Circulus arteriosus) (Fig. 434) is formed at the interpeduncular space of the base of the brain by the union of the anterior cerebral arteries in front, by the diverging posterior cerebral arteries behind, and is completed laterally by the junction of the latter with the posterior communicating arteries and by the internal carotid. It is irregularly polygonal in outline.

The cerebral arteries are very variable in arrangement, and the foregoing account is a brief statement of the more usual disposition of the larger vessels. The internal carotid artery often arises with the occipital by a common trunk of variable length.

THE EXTERNAL CAROTID ARTERY (Figs. 436, 437)

This artery (A. carotis externa) by its size and direction constitutes the continuation of the common carotid. It passes forward on the lateral wall of the pharynx at the lower border of the guttural pouch, covered by the submaxillary gland and the stylo-maxillaris, digastricus, and stylo-hyoideus muscles. It then emerges between the stylo-hyoideus and the great cornu of the hyoid bone, passes upward on the latter parallel with the posterior border of the lower jaw, and terminates about two inches (ca. 5 cm.) below the temporo-maxillary articulation by dividing into superficial temporal and internal maxillary branches. It is crossed deeply near its origin by the superior laryngeal and pharyngeal branches of the vagus nerve. Just before its emergence its superficial face is crossed by the hypoglossal nerve, and the glosso-pharyngeal nerve passes over its inner surface at the ventral border of the great cornu. The chief collateral branches are the masseteric, external maxillary, and posterior auricular. It also furnishes variable branches to the submaxillary and parotid glands, the guttural pouch, and the pharyngeal lymph glands, as well as twigs to some adjacent muscles.

1. The =inferior masseteric= or maxillo-muscular artery (A. masseterica inferior) is given off from the external carotid at its emergence from beneath the stylo-hyoideus. It passes downward and slightly forward under cover of the parotid gland and over the tendon of insertion of the sterno-cephalicus to the posterior border of the mandible and appears on the masseter muscle, into which it plunges after a short course on its surface. It gives branches also to the internal pterygoid and stylo-maxillaris muscles and the parotid gland.

2. The =external maxillary=, =facial=, or =submaxillary artery= (A. maxillaris externa) arises from the external carotid on the inner surface of the posterior belly of the digastricus (Fig. 436). It runs downward and forward on the lateral wall of the pharynx across the deep face of the stylo-hyoideus toward the great cornu of the hyoid bone, accompanied by the glosso-pharyngeal nerve in front and the hypoglossal nerve behind. After giving off the lingual artery at the posterior border of the great cornu, it inclines more ventrally on the inner surface of the internal pterygoid muscle, crosses over the hyo-glossus muscle, the hypoglossal nerve, the submaxillary duct, and the intermediate tendon of the digastricus, and turns forward in the submaxillary space. Here it lies on the lower part of the internal pterygoid muscle, and is related internally to the submaxillary lymph glands, above to the anterior belly of the digastricus, and below to the homonymous vein. At the anterior border of the masseter it turns around the lower border of the jaw and runs upward on the face in front of that muscle. At the turn the artery is in front, the vein in the middle, and the parotid duct posterior. The artery and vein pass upward along the anterior border of the masseter, under cover of the facial panniculus and the zygomaticus, and are crossed superficially by branches of the facial nerve and deeply by the parotid duct. The artery terminates over the levator labii superioris proprius by dividing into the dorsal nasal and the angular artery of the eye. The chief branches of the external maxillary are as follows:

(1) The =pharyngeal artery= (A. palatina ascendens) arises usually behind the stylo-pharyngeus, passes between that muscle and the great cornu, and runs forward on the lateral wall of the pharynx under the elastic pharyngeal fascia. It is distributed to the soft palate, pharynx, and tonsil.

FIG. 435.—SUPERFICIAL DISSECTION OF HEAD OF HORSE. MOST OF THE PANNICULUS IS REMOVED.

a, Dilatator naris superior; b, levator labii superioris proprius; c, levator nasolabialis; d, dilatator naris lateralis; e, buccinator; f, zygomaticus; g, depressor labii inferioris; h, stump of retractor anguli oris; i, masseter; k, k, scutularis; l, scutulo-auriculares superficiales; m, zygomatico-auricularis; n, parotido-auricularis; o, stylo-maxillaris; p, sterno-cephalicus; p′, tendon of p; q, omo-hyoideus; r, splenius; s, tendon of splenius and trachelo-mastoideus; t, corrugator supercilii; u, orbicularis oris; v, parotid gland; w, zygomatic arch; x, scutiform cartilage; y, upper commissure of nostril; 1, facial nerve; 2, superior buccal nerve; 3, inferior buccal nerve; 4, transverse facial nerve; 5, cervical branch of facial nerve; 6, posterior auricular branch of second cervical nerve; 7, cutaneous cervical branch of same; 8, infratrochlear nerve; 9, frontal nerve; 10, lacrimal nerve; 10′, end of auriculo-palpebral nerve; 11, inferior masseteric artery and vein; 12, transverse facial artery and vein; 13, facial artery; 14, inferior labial artery; 15, superior labial artery; 16, lateral nasal artery; 17, dorsal nasal artery; 18, angular artery of the eye; 19, posterior auricular artery; 20, 21, jugular vein; 22, external maxillary vein; 23, great auricular vein; 24, parotid duct; 24′, origin of same. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

(2) The =lingual artery= (A. lingualis) is a large branch which diverges from the parent trunk at an acute angle, runs along the ventral border of the great cornu of the hyoid bone, and dips under the hyo-glossus muscle. It then passes across the kerato-hyoideus, turns inward under the intercornual joint of the hyoid bone, and runs forward in the tongue between the hyo-glossus and genio-glossus. This part (A. profunda linguæ) is flexuous and is accompanied by branches of the hypoglossal and lingual nerves. It is the chief artery of the tongue, and anastomoses with the opposite artery and the sublingual.

(3) The =sublingual artery= (A. sublingualis) is a smaller vessel which arises at the anterior extremity of the submaxillary gland (Fig. 436). It passes forward on the anterior belly of the digastricus between the ramus of the mandible and the mylo-hyoideus, perforates the latter, runs along the lower border of the sublingual gland, and ramifies in the mucous membrane of the anterior part of the floor of the mouth. It detaches branches to the muscles and skin in the submaxillary space, the submaxillary lymph glands, and the sublingual gland. It also gives off the small =submental artery=, which runs forward superficially toward the lower lip, supplying twigs to the skin and the mylo-hyoideus.

FIG. 436.—PAROTID, MASSETERIC, AND LINGUAL REGIONS OF HORSE: DEEP DISSECTION, THIRD LAYER.

a, Mylo-hyoideus, anterior part, reflected; b, genio-hyoideus; c, genio-glossus; d, sublingual gland; e, ramus of mandible, the greater part of which is removed; e′, stump of masseter; f, maxillary tuberosity; g, great cornu of hyoid bone; h, wing of atlas; i, intermediate tendon of digastricus; i′, anterior belly, i″, posterior belly of digastricus; k, posterior part of mylo-hyoideus; l, hyo-glossus; m, pterygoideus internus (cut); n, stylo-hyoideus; o, stylo-maxillaris (cut); p, crico-pharyngeus; q, obliquus capitis ant.; r, tendon of trachelo-mastoideus; s, rectus cap. ant. major; t, mastoido-humeralis (cut); u, sterno-cephalicus (cut); v, sterno-thyroideus (cut); w, sterno-hyoideus (cut); x, omo-hyoideus; y, obliquus cap. post.; z, splenius (cut); 1–4, upper cheek teeth; 4′, last cheek tooth; 5, stump of facial nerve; 6, stump of buccinator nerve; 7, lingual nerve; 7′, superficial branch, 7″, deep branch of lingual nerve; 8, stumps of inferior alveolar artery, vein and nerve; 9, mylo-hyoid nerve (cut); 10, glosso-pharyngeal nerve; 11, hypoglossal nerve; 12, superior laryngeal nerve; 13, ventral branch of first cervical nerve; 14, vagus and sympathetic; 15, dorsal branch of spinal accessory nerve; 16, ventral branch of same; 17, inferior cerebral vein; 18, submaxillary duct; 19, common carotid artery; 20, parotid branch; 21, thyro-laryngeal artery; 22, pharyngeal artery; 23, laryngeal artery; 24, internal carotid artery; 25, occipital artery; 26, external carotid artery; 27–31, external maxillary artery; 28, posterior palatine artery; 29, lingual artery; 30, sublingual artery; 32, internal maxillary artery; 33, internal maxillary vein (origin); 34, remnant of parotid gland; 35, thyroid gland; 36, jugular vein; 37, pharyngeal lymph glands. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

In some cases the sublingual artery arises from the lingual and the submental from the external maxillary. Sometimes the sublingual remains on the external face of the mylo-hyoideus—thus resembling the submental of man—and the sublingual gland is supplied by a special branch of the lingual.

Not uncommonly a considerable branch, given off in the submaxillary space, turns round the lower border of the jaw and enters the middle of the lower part of the masseter muscle. In some cases this artery is of large size and its pulsation can be felt. It is accompanied by a vein.

(4) The =inferior labial artery= (A. labialis inferior) arises from the facial a little before it reaches the depressor labii inferioris (Fig. 435). It passes forward, dips under the depressor muscle, and continues to the lower lip. It supplies branches to the muscles and skin in this region, to the inferior buccal glands, the mucous membrane of the cheek, and the lower lip, anastomosing with the mental artery and the corresponding vessels of the opposite side. It detaches a branch (A. anguli oris) to the angle of the mouth, which anastomoses with the superior labial.