Part 67

d. =A. auricularis posterior= (Fig. 119, o).--The posterior auricular leaves the external carotid (m) opposite the middle of the bulla tympani and beneath the submaxillary gland. It passes about the base of the ear on its caudal and dorsal sides outside of the deep muscles of the occipital region, but beneath the auricular muscles, and sends several branches to the muscles of the external ear, passing onto the caudal surface of the concha. It sends also a large branch mediad to the muscles of the occiput, especially to the temporal muscle, within which it ramifies. A large branch (=anterior auricular=, Fig. 131, u) passes from the caudal side of the concha craniodorsad, and appears on the cranial side of the external ear, running along the cranial margin of the auditory opening.

e. =A. temporalis superficialis= (Fig. 119, p; Fig. 120, h).--The superficial temporal artery arises from the external carotid as the latter lies between the cartilaginous auditory meatus and the caudal border of the masseter muscle. It passes dorsad and gives off soon after its origin a muscular branch to the masseter, and an =auricular= branch which passes distad along the concha auris and ramifies over its cranial surface. The superficial temporal itself passes onto the surface of the temporal muscle (Fig. 120, 11), to which it gives numerous branches. It extends to the caudal angle of the eye (Fig. 131, s), where it divides. One branch passes into the lower eyelid; the larger branch passes along the dorsal side of the eye, sending a branch into the orbit and small branches onto the dorsal surface of the nose.

f. =A. maxillaris interna= (Fig. 119, n; Fig. 120, i).--The internal maxillary artery is the continuation of the external carotid. It turns caudad at the caudal end of the mandible, then passes craniad, lying dorsad of the pterygoid muscles (Fig. 120, 10), and against the medial surface of the mandible. It gives off the =inferior alveolar= artery (Fig. 120, j), then the =middle meningeal= (k), and then continuing mediad divides into three or four branches. The branches redivide, and the twigs form a complicated plexus, the =carotid plexus= (Fig. 120, l), which surrounds the maxillary division of the fifth nerve near its exit from the foramen rotundum. One of the larger branches of the plexus enters the skull through the orbital fissure, lying beside the hypophysis; it divides in the manner described below.

a, common carotid; b, branch to larynx; c, internal carotid; d, lingual; e, external carotid; f, posterior auricular; g, external maxillary; h, superficial temporal; i, internal maxillary; j, inferior alveolar; k, middle meningeal; l, carotid plexus; m, branch to temporal muscle; n, ophthalmic; o, infraorbital; p, lesser palatine. 1, M. constrictor pharyngis inferior; 2, M. sternothyreoideus; 3, M. sternohyoideus; 4, M. thyreohyoideus; 5, M. constrictor pharyngis medius; 6, M. hyoglossus; 7, M. geniohyoideus; 8, M. genioglossus; 9, M. digastricus; 10, Mm. pterygoidei externus and internus (cut); 11, M. temporalis (cut).]

Beyond the carotid plexus a main trunk which may be considered the continuation of the internal maxillary passes craniad, lying on the dorsal surface of the external pterygoid muscle; nearly opposite the molar tooth it divides into the =infraorbital= (Fig. 120, o) and the =sphenopalatine= arteries.

Branches of the internal maxillary artery and of the carotid plexus:

1. =A. alveolaris inferior= (Fig. 120, j).--The inferior alveolar (or inferior dental) artery leaves the inferior maxillary just opposite the condyloid process of the mandible. It enters the mandibular canal by the mandibular foramen, along with the nerve of the same name, and traverses the canal, furnishing branches to the lower teeth. It emerges at the mental foramen, and its terminal branches are distributed to the chin, but a branch is continued in the bone beyond the mental foramen and supplies the incisor and canine teeth of the lower jaw.

2. =A. meningea media= (Fig. 120, k).--The middle meningeal is a large vessel which leaves the internal maxillary at about the same level as the inferior alveolar. It passes into the foramen ovale and ramifies in the dura mater. Its branches leave distinct impressions on the inner surface of the bones of the skull.

3. A large branch (Fig. 121, h) from the plexus passes into the cranial cavity through the orbital fissure and lies within the skull at the side of the hypophysis. It gives off the following branches:

a. A =posterior communicating= branch, very short, which extends caudad and joins the internal carotid artery (Fig. 121, g).

b. =A. cerebri media= (Fig. 121, i).--The middle cerebral artery passes dorsad on the side of the cerebral hemisphere along the fissure of Sylvius and divides into numerous branches which are distributed to the surface of the cerebrum.

c. =A. cerebri anterior= (Fig. 121, j) Passes dorsad between the cerebral hemispheres. Just craniad of the optic chiasma the two anterior cerebral arteries are united by a small communicating branch, thus completing the circulus arteriosus or circle of Willis (Fig. 121), surrounding the hypophysis (see page 292).

4. From the carotid plexus several branches pass, arising either separately or in common, to the masseter, temporal, and pterygoid muscles.

5. =A. ophthalmica= (Fig. 120, n).--The ophthalmic artery passes from the carotid plexus to the structures in the orbit. It gives off numerous branches which supply the muscles of the eyeball, and other structures of this region. It sends an =ethmoidal= branch into the nasal cavity through the ethmoidal foramen in the orbital plate of the frontal bone, then continues distad to emerge from the orbit on the medial side of the eye; here it anastomoses with branches of the superficial temporal.

6. =A. palatina minor= (Fig. 120, p).--The lesser palatine artery leaves the internal maxillary distad of the carotid plexus, near the caudal border of the maxillary bone. It passes ventrocaudad into the soft palate.

7. =A. sphenopalatina.=--The sphenopalatine is one of the terminal branches of the internal maxillary; it passes mediad through the sphenopalatine foramen into the nasal cavity, and divides into numerous branches which supply the mucous membrane of the nose. It gives off just before it enters the sphenopalatine foramen the =descending palatine= (=A. palatina descendens=), which passes into the posterior palatine canal and emerges on the surface of the hard palate, where it ramifies.

8. =A. infraorbitalis= (Fig. 120, o).--The infraorbital artery is a direct continuation craniad of the internal maxillary. It sends off numerous small branches to the teeth of the upper jaw, and a rather large branch which passes to the lower eyelid. It then enters the infraorbital foramen, at the same time dividing usually into two or three branches; these emerge from the foramen and supply the parts of the nose and upper lip adjacent to the foramen (Fig. 131, r).

A. SUBCLAVIA. THE SUBCLAVIAN ARTERY.

The =left subclavian= (Fig. 118, c) arises from the convexity of the aortic arch just distad of the origin of the innominate artery, and about two or three centimeters from the heart. It passes craniad and slightly to the left, and turns into the left arm just craniad of the first rib.

The =right subclavian= (Fig. 115, n) is a direct continuation of the innominate, the artery receiving the name subclavian after the right common carotid is given off, usually at about the level of the second or third intercostal space.

The subclavian has the following branches: the =vertebral artery= (Fig. 118, g′), the =internal mammary= (Fig. 118, h), the =costocervical axis= (Fig. 118, g), the =thyrocervical axis= (Fig. 118, j). Beyond the last-named branch it continues into the arm as the =axillary= artery (Fig. 118, i; Fig. 122, g).

a, A. vertebralis; b, A. spinalis anterior; c, A. basilaris; d, A. cerebelli inferior posterior; e, A. cerebelli anterior; f, A. cerebri posterior; g, cut ends of the two internal carotid arteries; h, cut ends of branches from the carotid plexus (g and h are placed within the circulus arteriosus or circle of Willis); i, A. cerebri media; j, A. cerebri anterior.]

a. =A. vertebralis= (Fig. 118, g′).--The vertebral artery arises from the dorsal surface of the subclavian opposite the first rib. It passes craniad and dorsad at the side of the thoracic portion of the longus colli muscle, and enters the foramen transversarium of the sixth cervical vertebra. It passes thence craniad through the foramina transversaria (which together form the =vertebrarterial canal=) and gives off at the intervertebral foramina branches to the muscles of the neck and branches which pass across the ventral surface of the spinal cord to join A. spinalis anterior. Craniad of the foramen transversarium of the atlas the vertebral artery turns dorsad in the groove on the lateral surface of the atlas. Here it gives off a large branch which passes laterodorsad to the muscles of the neck and may anastomose with a branch of the occipital artery. The vertebral artery then passes into the vertebral canal through the atlantal foramen. It passes to the ventral side of the spinal cord and unites at about the level of the foramen magnum with the vertebral artery of the opposite side (Fig. 121, a) to form the =basilar artery (A. basilaris)= (Fig. 121, c), which passes craniad along the ventral middle line of the brain. Just before their union the two vertebral arteries (a) give off each a branch which passes caudomediad. These two branches soon unite in the middle line, forming the =anterior spinal artery= (=A. spinalis anterior=) (b), which passes caudad the entire length of the spinal cord, lying on its ventral middle line and receiving many communicating branches from the vertebral, intercostal, and lumbar arteries.

=A. basilaris= (c).--This arises by the union of the two vertebral arteries in the manner just described. It passes craniad along the ventral middle line of the medulla and pons, giving numerous small branches to these structures. A large branch, =A. cerebelli inferior posterior= (d), passes on each side to the caudal surface of the cerebellum, on which it ramifies. At the cranial margin of the pons the basilar artery divides. From each division a very large branch passes on each side across the pedunculi cerebri to the cranial part of the cerebellum; this is =A. cerebelli anterior= (e). Just craniad of this, arising from nearly the same point, the smaller =A. cerebri posterior= (f) passes laterad to the caudal part of the cerebrum; it is joined by the internal carotid artery (g). The small continuations of the two halves of =A. basilaris= then pass craniad at the side of the hypophysis to join the posterior communicating branches from the carotid plexus. As other branches of the carotid plexus unite across the middle line craniad of the optic chiasma, an arterial circle is formed on the base of the brain, surrounding the hypophysis and the optic chiasma. This is known as the =circulus arteriosus=, or circle of Willis (see page 289).

b. =A. mammaria interna= (Fig. 118, h).--The internal mammary artery rises from the ventral surface of the subclavian opposite the first rib, and passes in the mediastinum ventrocaudad to reach the sternum opposite the third intercostal space. It sends two or three small branches craniad to the midventral part of the thoracic wall and then extends caudad at the side of the sternum. It sends off lateral branches to the ventral thoracic wall, branches to the mediastinum and pericardium, a branch to the diaphragm which anastomoses with the phrenic; and finally it passes out of the thoracic cavity caudad of the last costal cartilage, extends caudad in a zigzag course at the lateral border of the rectus muscle and anastomoses with the inferior epigastric artery.

c. =Truncus costocervicalis= (Fig. 118, g).--The costocervical axis arises from the subclavian opposite the first rib, passes craniodorsad, and divides almost at once into two branches. The smaller one of these, the =superior intercostal= (=A. intercostalis suprema=), passes caudad and, dividing, supplies the first and second intercostal spaces, and then passes to the deep muscles of the back. The other branch divides almost immediately into two. One of these, =A. transversa colli=, passes laterad in front of the first rib and enters the serratus anterior muscle. In this it passes dorsad, giving off branches to this muscle and to the levator scapulæ, till it reaches M. rhomboideus, which it likewise supplies. The other branch (=A. cervicalis profunda=), which seems to form a continuation of the main artery, passes directly dorsad and leaves the thoracic cavity between the heads of the first and second ribs. Here it passes into the deep muscles of the neck; it can be traced in the substance of the complexus muscle as far forward as the atlas.

d. =Truncus thyrocervicalis= (Fig. 118, j).--The thyrocervical axis (or thyroid axis) arises from the subclavian beneath the first rib, a short distance distad of the origin of the costocervical axis. It passes laterocraniad and dorsad, lying on the mediocranial side of the brachial plexus. A short distance from its origin it gives off a branch, varying much in size, the =cervicalis ascendens= (Fig. 119, d), which passes craniad on the ventral side of the neck, supplying the sternomastoid, sternohyoid, the cervical portion of the scalenus, and sometimes other muscles of the neck region. Other branches pass from the thyroid axis to the inner surface of the clavotrapezius (Fig. 122, a) and to the ventral end of the pectoral muscles. At the level of the cranial border of the scapula the artery takes the name =A. transversa scapulæ=, or =suprascapularis=. This divides into three main branches. The first branch, sometimes large, sometimes small, passes to the lymphatic gland in the hollow of the shoulder, and to the adjacent muscles; it supplies the clavotrapezius, acromiotrapezius, levator scapulæ ventralis, splenius, occipitoscapularis, and rhomboideus. A second branch passes between the subscapularis and supraspinatus muscles, dividing into various branches which supply the muscles named. The third branch pierces the supraspinatus muscle, just craniad of the acromion process, and ramifies in that muscle.

e. =A. axillaris= (Fig. 122, g).--The axillary artery is the continuation of the subclavian laterad of the first rib. It lies caudad of the brachial plexus and parallel to it. It sends off the following branches: