Part 26

The pulmonary artery passes craniodorsad and slightly to the left from the cranial end of the conus arteriosus. One to one and a half centimeters from the conus it divides into right and left branches (Fig. 116, _j_). Just before the division the dorsal surface of the pulmonary artery is connected by the short =ligamentum arteriosum=, or ligamentum Botalli, with the aorta. This is the remnant of a canal which in fœtal life forms a free communication between the pulmonary artery and the aorta; this canal is known as the =ductus Botalli=. The ligament is almost or quite obliterated in the adult cat.

The left branch of the pulmonary artery passes to the left lung, crossing ventrad of the thoracic aorta. It then divides into branches which pass to the lobes of the left lung. The point of division of the left branch of the pulmonary artery lies craniad of all the lobes of the lung, so that the lobes of the left lung are said to be all =hyparterial=, i.e., below (or caudad of) the artery.

The right branch passes under (dorsocaudad of) the aortic arch, and reaches the right lung at about the junction of the cranial lobe with the remainder of the lung. The cranial lobe of the right lung is therefore said to be =eparterial=, since it is craniad of the pulmonary artery; the other lobes are hyparterial. The right branch divides at its entrance to the lung and is distributed to its lobes.

2. =Aorta= (Fig. 115, _g_; Fig. 118, _a_).

The aorta is the single great vessel which conveys blood from the left ventricle. It makes a sharp semicircular curve dorsad (Fig. 118, _a′_) and to the left, passes caudad at the left side of the vertebral column, and passes between the crura of the diaphragm to reach the abdominal cavity. It is divisible into =thoracic aorta= (Fig. 118) and =abdominal aorta= (Fig. 126).

A. =Thoracic Aorta= (Fig. 118).--The first portion of the thoracic aorta, curved as above described, is the =aortic arch= (_a′_). It lies in the thoracic cavity opposite the interval between the third and fourth or fourth and fifth ribs. It is separated from the vertebral column on the right by the superior vena cava (_q_), and on the left by the œsophagus (4). At its beginning it lies a little to the right of the median plane; but it passes at once to the left side of the vertebral column.

BRANCHES OF THE THORACIC AORTA.

1. =Aa. coronariæ.=--The coronary arteries are two. They arise from the aortic sinuses. The left one (Fig. 118, _p_) leaves the aorta on the left side, passes dorsad of the pulmonary artery, and divides into two branches, one of which follows the auriculoventricular groove (sulcus coronarius) to the dorsal side of the heart and sends branches to the adjacent heart-walls, while the other runs onto the ventricles, following approximately the ventral border of the ventricular septum. The _right coronary_ artery passes in the auriculoventricular groove toward the right and dorsad, and supplies the adjacent walls of the heart.

2. =A. anonyma= (Fig. 115, _j_; Fig. 118, _b_).--The innominate artery passes craniad from the convexity of the aortic arch (_a′_). It gives rise first to a small =mediastinal artery= (_m_) which passes ventrad into the mediastinum, then to the =left common carotid= (Fig. 115, _l_), then to the =right common carotid= (Fig. 115, _m_). Sometimes the carotids are given off from a common trunk. Beyond the right common carotid the innominate is continued as the =right subclavian= (Fig. 115, _n_).

3. =A. subclavia sinistra= (Fig. 115, _k_; Fig. 118, _c_).--The left subclavian passes craniad from the convexity of the aortic arch. It curves about the first rib (Fig. 118, _I_) to enter the armpit, where it becomes the =axillary artery= (_i_).

4. =Aa. intercostales= (_n_).--The intercostal arteries are given off from the dorsal side of the aorta. Each passes to an intercostal space and divides into three branches. One of these passes ventrad along the caudal margin of the cranial one of the two ribs between which it extends. Another goes to the deep muscles of the back, and the third enters the spinal canal through the intervertebral foramen. The arteries for the first and second (and sometimes the third) intercostal space usually arise from the subclavian (costocervical axis, Fig. 118, _g_).

5. =Aa. bronchiales.=--The bronchial arteries are two, and arise either from the aorta opposite the fourth intercostal space or from the fourth intercostal arteries. They accompany the bronchi to the lungs.

6. =Aa. œsophageæ= (Fig. 118, _o_).--The œsophageal arteries are small branches of varying origin passing to the œsophagus.

7. =Aa. lumbales.=--The lumbar arteries correspond to the intercostals, but pass off between the lumbar vertebræ. The obliquity of the diaphragm throws the origin of one or two pairs of them within the thorax.

A. CAROTIS COMMUNIS. THE COMMON CAROTID ARTERY. (Fig. 115, _l_ and _m_; Fig. 119, _a_).

The two common carotid arteries arise from the innominate artery in the manner already described. Each passes craniad along the side of the trachea. In the thorax (Fig. 118, _f_) the common carotid lies mediad of the subclavian artery (Fig. 118, _c_) and dorsad of the superior vena cava (Fig. 118, _q_). In the neck (Fig. 119) the artery lies, accompanied by the vagus and sympathetic nerves and the internal jugular vein (_b_), in the space between the longus capitis muscle (7) and the trachea (14); it is covered ventrally by the sternomastoid and sternothyroid (2) muscles, lying close to the lateral border of the latter. Near its origin the common carotid may give rise to the small =inferior thyroid= artery. It then passes to about the level of the larynx without giving off branches; here it gives off the =superior thyroid= (_c_) on the ventral side and one or more muscular branches (_e_) on the dorsal side. One or two centimeters further craniad it gives off on the dorsal side the =internal carotid= (_g_) and the occipital artery (_f_). The main artery now takes the name =external carotid= (_m_).

Branches of the common carotid:

1. =A. thyreoidea ima.=--The inferior thyroid is a small artery which arises either from the common carotid near its origin, or from the innominate before the origin of the carotid. It passes craniad on the trachea as far as the thyroid gland, giving branches to the trachea and œsophagus.

2. =A. thyreoidea superior= (_c_).--The superior thyroid leaves the carotid opposite the thyroid cartilage and passes mediad and caudad, sending branches to the thyroid gland, and the sternothyroid (2) and sternohyoid (1) muscles. A small branch, the =superior laryngeal=, passes to the larynx, and supplies those muscles of the larynx which are not enclosed by the cartilages.

3. =Rami musculares= (_e_).--One or two branches, usually of considerable size, leave the common carotid at about the same level as the superior thyroid and pass to the muscles on the dorsal side of the neck, the main trunk of the artery passing between the longus capitis (7) and scalenus muscles (10).

4. =A. occipitalis= (_f_).--The occipital artery arises from the common carotid at about the same point as the internal carotid. It immediately sends a large branch dorsad, passing between M. longus capitis (7) and the vertebral column, to the deep muscles of the neck. The occipital then crosses the outer surface of the digastric muscle (8) to the back of the skull, and runs along the lambdoidal crest just beneath the splenius muscle. It sends a number of branches to the muscles of the back of the neck; and one of its branches may unite with the vertebral artery as it lies in the groove on the atlas, or with a branch of the vertebral.

5. =A. carotis interna= (_g_).--The internal carotid artery is one of the terminal branches of the common carotid. It is very small. It is given off near or in common with the occipital artery, passes toward the cranial end of the tympanic bulla, enters the bulla with the Eustachian tube, and passes into the skull at the foramen lacerum. Its course is much convoluted before entering the foramen. Within the skull (Fig. 121, _g_) it joins the posterior cerebral artery (Fig. 121, _f_) at the side of the hypophysis.

6. =A. carotis externa= (Fig. 119, _m_).--After giving off the internal carotid the continuation of the common carotid artery receives the name =external carotid= (_m_). It passes craniad and laterad between the digastric (8) and styloglossus muscles, where it gives off cranioventrad the =lingual artery= (_i_) and a number of small muscular branches; also sometimes the small =laryngeal= artery. At the dorsolateral border of the digastric (8) it gives off the =external maxillary= artery (_j_), and about one centimeter farther craniad the =posterior auricular= (_o_). It now turns mediad, lying against the cartilaginous auditory meatus, on its cranial side, and gives off the =superficial temporal= (_p_). The artery then continues mediad, taking the name =internal maxillary= (_n_)--so that the internal maxillary artery is to be considered the terminal branch of the external carotid.

Branches of the external carotid artery (Fig. 119):

_a._ =A. lingualis= (Fig. 119, _i_; Fig. 120, _d_).--The lingual artery leaves the external carotid near its beginning and passes craniomediad along the ventral border of the digastric muscle (Fig. 120, 9), accompanied by the hypoglossal nerve. It gives off numerous small branches to the hyoid and pharyngeal muscles, then passes dorsad of the hyoglossus muscle (Fig. 120, 6), where it gives off a branch which passes transversely across the middle line to communicate with the artery of the other side. Beneath the hyoglossus the artery turns craniad and passes into the tongue. Here it runs along the medial border of the styloglossus to the tip of the tongue, giving off numerous branches into the substance of this organ.

_b._ =Rami musculares.=--Muscular branches pass to the digastric and to the hyoid muscles. A small branch (Fig. 119, _h_), which may arise either from the external carotid or from the common carotid near the beginning of the external carotid, passes to the larynx and supplies the thyroarytenoid and lateral cricoarytenoid muscles of the larynx.

_c._ =A. maxillaris externa= (Fig. 119, _j_).--The external maxillary artery leaves the external carotid (_m_) opposite the angle of the jaw and at about the dorsal border of the digastric muscle (8). It passes craniad, lying at first beneath the digastric muscle and sending a branch to the submaxillary gland. Opposite the caudal border of the mylohyoid muscle it gives off the =submental= artery, turns dorsad, emerges from beneath the digastric, and passes along the cranial border of the masseter onto the face. Here it divides into =superior= (_k_) and =inferior= (_l_) =labial= branches, which pass along the upper and lower lips, respectively, giving off numerous branches.

The =submental= artery passes to the symphysis menti between the digastric and mylohyoid muscles, giving off on its course collateral branches to the muscles of this region.

_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.

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= (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.