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the aortic arch, to which it is connected by the ligamentum arteriosum, and the left recurrent nerve; below, it is in relation with the lower left pulmonary vein. After entering the lung it descends, like the right pulmonary artery, posterior and lateral to the stem bronchus, and between its ventral and dorsal branches.

Branches. Just before it passes through the hilum it gives off a branch to the upper lobe of the left lung, and in the substance of the lung its branches correspond with the ventral, dorsal, and accessory branches of the bronchial tube.

THE SYSTEMIC ARTERIES.

AORTA.

The aorta is the main trunk of the general arterial system. It commences at the base of the left ventricle and ascends, with an inclination to the right, to the level of the second right costal cartilage; then it curves backwards and to the left, until it reaches the left side of the lower border of the fourth thoracic vertebra; there it turns downwards and descends, through the thorax into the abdomen, where it terminates, on the left of the median plane, at the level of the fourth lumbar vertebra, by bifurcating into the two common iliac arteries. The portion of the aorta which is situated in the thorax is, for convenience, termed the thoracic aorta, and the rest of the vessel is known as the abdominal aorta.

AORTA THORACALIS.

The thoracic aorta is subdivided into aorta ascendens, arcus aortæ, and aorta descendens.

Aorta Ascendens.-The ascending aorta lies in the middle mediastinum. It springs from the base of the left ventricle, posterior to the left margin of the sternum, opposite the lower border of the third left costal cartilage and at the level of the body of the sixth thoracic vertebra. From its origin it passes upwards, anteriorly, and to the right, and it terminates in the arch of the aorta, posterior to the right margin of the sternum, at the level of the second costal cartilage. Its length is from 50 to 56 mm. (2 to 24 inches), and its diameter is 28 mm. (1 inches). In the adult it is a little narrower at its commencement than the pulmonary artery is, but in old age it enlarges and exceeds the latter vessel in size. The diameter, however, is not uniform throughout the whole length of the ascending aorta. Its dilated commencement, the bulbus aortæ, has three secondary dilatations, the sinus aorta (Valsalva) in its wall, immediately above the semilunar cusps of the aortic valve; one is anterior in position, and two are situated posteriorly. At a higher level there is a diffuse bulging of the right wall, which is known as the great sinus of the aorta.

Relations. The ascending aorta is completely enclosed within the fibrous pericardium which blends above with the sheath of the vessel, and it is enveloped, together with the stem of the pulmonary artery, in a tubular prolongation of the serous pericardium. At its origin it has the pulmonary artery in front, the transverse sinus of the pericardium and the anterior wall of the left atrium behind, and the right atrium on its right side. In the upper part of its course the ascending aorta is overlapped by the anterior margins of the right lung and right pleural sac, whilst posterior to it are the right atrium, the right branch of the pulmonary artery, the right bronchus, and the left margin of the superior vena cava. The superior vena cava lies on the right side, and partly posterior to the upper part of the ascending aorta, whilst the pulmonary artery is at first anterior to it and then, at a higher level, on its left side.

Branches. Two branches arise from the ascending aorta, viz., the right and the left coronary arteries. The right coronary artery springs from the anterior, and the left from the left posterior sinus of the aorta (Valsalva) (Fig. 751).

Arcus Aorta.-The arch of the aorta lies in the superior mediastinum, posterior to the lower part of the manubrium sterni, and connects the ascending with the descending aorta. It commences posterior to the right margin of the sternum, on a level with the second costal cartilage, and extends to the left side of the lower

border of the fourth thoracic vertebra. As its name implies, it forms an arch; and the arch makes two curves, one with the convexity upwards, and the other with the convexity forwards and to the left. From its origin it runs for a short distance upwards, posteriorly, and to the left, anterior to the trachea; then it passes posteriorly, round the left side of the trachea to the left side of the body of the fourth thoracic vertebra. Finally it turns downwards to become continuous with the descending aorta.

At its commencement it has the same diameter as the ascending aorta, 28 mm. (1 inches), but after giving off three large branches, the diameter is reduced to 23 mm. (a little less than one inch).

Relations. It is overlapped anteriorly and on the left side by the right and left lungs and pleural sacs, but much more by the left than the right, and in the interval between and posterior to the anterior borders of the pleural sacs it is covered by the remains of the thymus. As it turns backwards it is crossed vertically, on the left side, by four nerves in the following order from before backwards: the left phrenic, the inferior cervical cardiac branch of the left vagus, the superior cardiac branch of the left sympathetic, and the trunk of the left vagus. The left superior intercostal vein passes obliquely upwards and to the right, across it, between the left vagus and left phrenic nerves.

Posterior to, and to the right side of the arch, are the trachea, the deep cardiac plexus, the left recurrent nerve, the left border of the œsophagus, and the thoracic duct. Above are its three large branches-the innominate, the left common carotid, and the left subclavian arteries; and crossing anterior to their roots is the left innominate vein. Below is the bifurcation of the pulmonary artery and the root of the left lung; the ligamentum arteriosum, which is also below, attaches it to the commencement of the left pulmonary artery, whilst to the right of the ligament lies the superficial cardiac plexus, and to its left the left recurrent nerve.

Branches. The three great vessels which supply the head and neck, part of the thoracic wall, and the upper extremities-viz. the innominate, the left common carotid, and the left subclavian arteries-arise from the aortic arch.

Aorta descendens. The thoracic portion of the descending aorta lies in the posterior mediastinum; it extends from the termination of the arch, at the lower border of the left side of the fourth thoracic vertebra, to the aortic opening in the diaphragm, where, opposite the twelfth thoracic vertebra, it becomes continuous with the abdominal portion. Its length is from 17.5 to 20 cm. (seven to eight inches), and its diameter diminishes from 23 mm. at its commencement to 21 mm. at its termination.

Relations. Immediately posterior to it are the vertebral column and the anterior longitudinal ligament. It rests also on the accessory hemiazygos and the hemiazygos veins, whilst from its posterior aspect the aortic intercostal branches are given off.

Anteriorly it is in relation, from above downwards, with the root of the left lung, the pericardium, which separates it from the back of the left atrium, the oesophagus with the oesophageal plexus of nerves, and the crura of the diaphragm which separate it from the caudate lobe of the liver. On the left side are the left lung and pleura. On the right side the thoracic duct and the vena azygos form immediate relations along its whole length. The œsophagus also lies to the right of the upper part of the descending aorta, whilst the right lung and pleura are in relation below.

Branches. Nine pairs of aortic intercostal arteries, two left bronchial arteries, four or five œsophageal, some small pericardial, and a few posterior mediastinal and superior phrenic branches, usually arise from the thoracic part of the descending aorta.

AORTA ABDOMINALIS.

The abdominal portion of the descending aorta lies in the epigastric and umbilical regions of the abdomen. It extends from the middle of the lower border of the last thoracic vertebra to the body of the fourth lumbar vertebra, where, to the left of the median plane, it bifurcates into the right and left common iliac arteries. The point of division is a little below and to the left of the umbilicus, opposite

a line drawn transversely across the abdomen on a level with the highest points of the iliac crests.

At its commencement it is 21 mm. in diameter, but after the origin of two large branches, the coeliac and the superior mesenteric arteries, it diminishes considerably, and then retains a fairly uniform diameter to its termination.

Relations. Posteriorly, it is in contact with the upper four lumbar vertebrae and intervening fibro-cartilages, the anterior longitudinal ligament, and the left lumbar veins ;

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FIG. 758.-THE ABDOMINAL AORTA AND ITS BRANCHES IN A FORMALIN HARDENED PREPARATION,

the lumbar and the middle sacral arteries spring from the posterior surface of the vessel. Anteriorly, and in close relation with it, there are from above downwards the following structures: the coeliac axis and coeliac plexus, the pancreas and splenic vein, the superior mesenteric artery, the left renal vein, the third part of the duodenum, the root of the mesentery, the aortic plexus, the inferior mesenteric artery, the peritoneum and coils of small intestine. More superficially the stomach, the transverse colon, and the greater and lesser omenta are in front. On the right side, in the upper part of its extent, are the thoracic duct and cisterna chyli, the vena azygos, and the right crus of the diaphragm, the latter separating it from the right coeliac ganglion and from the upper part of the

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inferior vena cava. Its lower part is in direct relation, on the right side, with the inferior vena cava. On the left side, the left crus of the diaphragm with the left coeliac ganglion, and the terminal portion of the duodenum, are in close relation with its upper part, whilst in the lower portion of its extent the peritoneum and some coils of the small intestine are in contact with it. Lumbar lymph glands lie around it, on all sides.

Branches. The branches form two groups, visceral and parietal, and each group consists of paired and unpaired vessels, as follows:

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Unpaired.

Middle sacral (which Inferior phrenic
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Paired.

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ovarian

Lumbar (four pairs)
Common iliac

BRANCHES OF THE ASCENDING AORTA.

ARTERIA CORONARIÆ.

The coronary arteries are two in number, a right and a left; they are distributed almost entirely to the heart, but give also some small branches to the roots of the great vessels, and to the pericardium (Figs. 750, 751, and 754).

The right coronary artery springs from the anterior aortic sinus. It runs forwards, between the root of the pulmonary artery and the auricle of the right atrium, to the coronary sulcus, in which it passes downwards and to the right to the junction of the right and inferior margins of the heart. There it turns to the left, in the inferior part of the coronary sulcus, as far as the posterior end of the inferior interventricular sulcus, where it gives off its interventricular branch and then ends by anastomosing with the circumflex branch of the left coronary artery. It is accompanied by branches from the cardiac plexus and the right coronary vein.

Branches.-The interventricular branch runs forwards in the inferior interventricular sulcus; it supplies both ventricles, and anastomoses, at the apex of the heart, with the interventricular branch of the left coronary artery.

Aortic and pulmonary twigs are distributed to the roots of the aorta and pulmonary artery respectively. A right atrial branch passes upwards on the anterior surface of the right atrium, between it and the ascending aorta; one or more anterior ventricular branches, of small size, descend on the anterior surface of the right ventricle; a branch of larger size, the right marginal artery, runs along the inferior margin of the heart and gives branches to both surfaces of the right ventricle.

The left coronary artery arises from the left posterior aortic sinus. Its short trunk runs forwards, between the root of the pulmonary artery and the auricle of the left atrium, to the coronary sulcus at the upper end of the anterior interventricular groove, where it divides into a circumflex and an interventricular branch.

Branches. The circumflex branch runs to the left margin of the heart, and there turns to the inferior surface where it comes into relation with the coronary sinus; it ends by anastomosing with the right coronary artery. It supplies branches to the left atrium, the left margin of the heart, and the posterior part of the inferior surface of the left ventricle. The interventricular terminal branch passes down the anterior interventricular sulcus to the apex of the heart, where it anastomoses with the interventricular branch from the right coronary; it supplies both ventricles, and is accompanied by cardiac nerves and by the great cardiac vein.

A left atrial branch or branches of small size pass to the wall of the left atrium, and small aortic and pulmonary branches are also given to the roots of the aorta and pulmonary artery.

BRANCHES OF THE ARCH OF THE AORTA.

The branches which arise from the arch of the aorta supply the head and neck, the upper extremities, and part of the body wall.

They are three in number, viz., the innominate, the left common carotid, and the left subclavian arteries. The innominate is a short trunk, from the termination of which the right common carotid and the right subclavian arteries spring (Figs. 756 and 757); thus there is, at first, a difference between the stem vessels of opposite sides, but the subsequent course and the ultimate distribution of those vessels closely correspond.

ARTERIA ANONYMA.

The innominate artery (Fig. 757) arises, posterior to the middle of the manubrium sterni, from the convexity of the arch of the aorta near its right or anterior extremity, and it ends opposite the right sterno-clavicular articulation, where it divides into the right subclavian and right common carotid arteries.

Course. The trunk measures from 37 to 50 mm. in length; it runs upwards, posteriorly, and laterally, in the superior mediastinum, to the root of the neck.

Relations.-Posterior.-It is in contact behind, with the trachea below and with the right pleural sac above.

Anterior. The left innominate vein crosses in front of the lower part of the artery, and above that the sterno-thyreoid muscle separates it from the sterno-hyoid and the right sterno-clavicular joint. The remains of the thymus, which separate it from the manubrium sterni, are also in front.

Right Lateral.-The right innominate vein and the upper part of the superior vena cava are on the right side of the artery.

Left Lateral.-On its left side is the origin of the left common carotid artery, whilst at a higher level the trachea is in contact with it.

Branches. As a rule the innominate artery does not give off any branches except its two terminals, but occasionally it furnishes an additional branch, the thyreoidea ima.

The thyreoidea ima is an inconstant and slender vessel. When present it may arise from the arch of the aorta, but it springs usually from the lower part of the innominate. It passes upwards, anterior to the trachea, through the anterior part of the superior mediastinum and the lower part of the neck, and gives off branches to the lateral lobes and isthmus of the thyreoid body and to the trachea.

THE ARTERIES OF THE HEAD AND NECK.

The vessels distributed to the head and neck are chiefly derived from the carotid trunks; there are, however, in addition, other vessels which arise from the main arterial stems of the upper extremities, and it will be advantageous to describe the most important of those, viz., the vertebral arteries, with the carotid system. The smaller additional branches will be considered along with the remaining branches of the subclavian arteries.

The carotid system of arteries consists, on each side, of a common carotid trunk, which divides into internal and external carotid arteries, from which numerous branches are given off (Figs. 759, 760, 761, 764).

The internal carotid arteries are distributed, almost entirely, to the contents of the cranial cavity, internal to the dura mater, and to the structures in the cavity of the orbit. The external carotid arteries, on the other hand, supply structures of the head and neck more externally situated.

It is to be noted, however, that the vascular supply of the brain is not wholly derived from the internal carotid vessels, but that it is contributed to, largely, by the vertebral arteries also.

ARTERIE CAROTIDES COMMUNES.

The right and the left common carotid arteries are of unequal length. The right common carotid commences at the bifurcation of the innominate artery posterior to the right sterno-clavicular articulation; the left arises in the superior mediastinum, from the arch of the aorta; but each terminates at the level of the upper border of the thyreoid cartilage; the left artery has thus a short intra

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