vasorum. The vasa vasorum or vasa nutritia are distributed in the external coat, some few making their way among the external layers of the middle coat. They are met with even on the smallest arteries, and form a rich capillary netI work with rounded meshes. The small veins which return the blood from the capillaries, open into the companion veins of the artery. The nerves, like the vasa vasorum, are found only in the external coat, and are wanting altogether in many arteries, as in those of the cerebral and spinal substance, the choroid membrane of the ovum, the placenta, and also in the arteries of some muscles, glands, and membranes.

In the consideration of the arteries, we shall first describe the aorta, and the branches of that trunk with their subdivisions, which together constitute the efferent portion of the systemic circulation; and then the pulmonary artery as the efferent trunk of the pulmonary circulation.

AORTA.

The AORTA arises from the base of the left ventricle, at the middle of the root of the heart, and presents at its commencement an enlargement (bulbus aorta) caused by three dilatations of the walls of the vessel, the sinus aortici (sinus Valsalvæ) and corresponding with the three semilunar valves. It ascends at first forwards and to the right, then curves backwards and to the left, and descends on the left side of the vertebral column to the fourth lumbar vertebra. Hence it is divided into the arch and descending aorta.

The ARCH OF THE AORTA (arcus aorta), commencing at a point corresponding with the articulation of the cartilage of the fourth rib with the sternum on the left side, crosses behind and near the sternum to a point corresponding with the upper border of the articulation of the second rib with the sternum on the right side. It then curves backwards and to the left, and descends to the left side of the body of the third dorsal vertebra, and at the lower border of the latter vertebra becomes the thoracic aorta.

The first or ascending portion of the arch (aorta ascendens), a little more than two inches in length, is almost wholly contained within the pericardium. It is crossed in front by the pulmonary artery; on its left side it has the left auricle and pulmonary artery: on its right the right auricle and superior vena cava; and behind the right pulmonary artery and veins.

The second or transverse portion of the arch is crossed in front by the left phrenic nerve, left nervus superficialis cordis, left inferior cardiac of the pneumogastric, and left pneumogastric nerve. Behind it is in relation with the trachea, oesophagus, thoracic duct, nerves to the deep cardiac plexus, and left recurrent nerve. Above it gives off the arteria innominata, left carotid and left subclavian artery, and supports the left vena innominata; and below is in relation with the superficial cardiac plexus, bifurcation of the pulmonary artery, cord of the ductus arteriosus, left bronchus, and left recurrent nerve.

The third or descending portion of the arch lies against the third dorsal vertebra, and is partially covered by the left pleura.

AORTA DESCENDENS. - The descending aorta is subdivided in correspondence with the two great cavities of the trunk, into the thoracic and abdominal aorta. Aorta thoracica.-The thoracic aorta commencing at the lower border and left side of the third dorsal vertebra curves gently towards the right as it descends, and as it passes through the aortic opening of the diaphragm lies on the middle line of the vertebral column.

It is in relation behind, with the vertebral column and lesser vena azygos; in front, with the oesophagus and right pneumogastric nerve; to the left side, with the pleura; and to the right with the thoracic duct.

Aorta abdominalis. - The abdominal aorta enters the abdomen through the aortic opening of the diaphragm, and between the two pillars of that muscle. In

THE LARGE VESSELS OF THE ROOT OF THE HEART AND LUNGS. 1. Ascending aorta; the numeral is placed on the dilatation known as the sinus quartus vel maximus Valsalvæ. 2. Transverse portion of the arch. 3. Thoracic portion of the descending aorta. 4. Arteria innominata, dividing into, 5, the right common carotid, which again divides at 6, into the external and internal carotid; and 7, the right subclavian artery. 8. Axillary artery; its extent is designated by a dotted line. 9. Brachial artery. 10. Right pneumogastric nerve running by the side of the common carotid in front of the right subclavian artery, and behind the root of the right lung. 11. Left common carotid, having on its outer side the left pneumogastric nerve, which crosses the arch of the aorta, and as it reaches its lower border gives off the left recurrent nerve. 12. Left subclavian artery becoming axillary and brachial in its course, like the artery of the opposite side. 13. Trunk of the pulmonary artery connected to the concavity of the arch of the aorta by a fibrous cord, the remains of the ductus arteriosus. 14. Left pulmonary artery. 15. Right pulmonary artery. 16. Trachea. 17. Right bronchus. 18. Left bronchus. 19, 19. Pulmonary veins. 17, 15, 19, on the right side, and 14, 18, 19, on the left, constitute the roots of the corresponding lungs, and the relative position of the vessels is preserved. 20 Bronchial arteries. 21, 21. Intercostal arteries; the branches from the front of the aorta above and below the number 3 are pericardiac and œsophageal.

its course downwards, it lies on the vertebral column to the left of the middle line, and terminates on the fourth lumbar vertebra, by dividing into the two common iliac arteries. It is crossed in front by the left renal vein, pancreas, transverse duodenum, and mesentery, and is in relation behind with the thoracic duc, receptaculum chyli, and left lumbar veins. On its left side is situated the left semilunar ganglion, supra-renal capsule, and sympathetic nerve; on its right, the inferior vena cava, right semilunar ganglion, and commencement of the vena azygos.

The CORONARY ARTERIES (cardiaca) arise from the aortic sinuses at the commencement of the ascending portion of the arch of the aorta, immediately 9bove the free margin of the semilunar valves.

The left coronary (coronaria cordis sinistra) passes forwards, between the pulmonary artery and left appendix auriculæ, and divides into two branches; one of which (posterior) winds around the base of the left ventricle in the auriculoventricular groove, and inosculates with the right coronary, forming an arterial circle around the base of the heart; the other (anterior) passes along the groove of union of the two ventricles, on the anterior aspect of the heart, to its apex, where it anastomoses with the right coronary. The left coronary artery supplies the left auricle and anterior surface of both ventricles.

The right coronary (coronaria cordis dextra) passes forwards, between the root of the pulmonary artery and the right auricle, and winds along the auriculoventricular groove, to the posterior longitudinal furrow; it then descends along

the posterior aspect of the heart to its apex, and inosculates with the left coronary. It is distributed to the right auricle and ventricle, and posterior surface of both ventricles, and sends a large branch along the sharp margin of the right ventricle to the apex of the heart.

ARTERIA INNOMINATA.

The Arteria innominata [brachio-cephalic artery] (fig. 209, ) is the first and largest artery given off by the arch of the aorta. It is an inch and a half in length, and ascends obliquely towards the right sterno-clavicular articulation, where it divides into the right carotid and right subclavian artery.

Relations.

-It is in relation, in front, with the left vena innominata, sternum, and origin of the sterno-hyoid and sterno-thyroid muscle; behind, with the trachea and right pneumogastric nerve; to the right with the right vena innominata and pleura; to the left, with the left common carotid artery and remains of the thymus gland.

Plan of the Relations of the Arteria Innominata.

The arteria innominata occasionally gives off a small branch, which ascends along the middle of the trachea to the thyroid gland. This branch was described by Neubauer, as the thyroidea ima (lowest thyroid artery), and Dr. Harrison names it the middle thyroid artery. A knowledge of its existence is important in performing the operation of tracheotomy.

COMMON CAROTID ARTERIES.

The common carotid arteries (carotis communis vel primitiva; xápa, the head) arise, the right from the bifurcation of the arteria innominata opposite the right sterno-clavicular articulation, the left from the arch of the aorta. It follows, therefore, that the right carotid is shorter than the left; it is also more anterior; and, in consequence of proceeding from a branch instead of from the main trunk, is larger than its fellow.

The Right common carotid artery, (fig. 209, 5) ascends the neck perpendicularly by the side of the trachea and larynx, from behind the right sterno-clavicular articulation to a level with the upper border of the thyroid cartilage, where it divides into the external carotid and internal carotid.

The Left common carotid (fig. 209, 11) passes somewhat obliquely outwards from the arch of the aorta to the side of the neck, and thence upwards by the side of the trachea and oesophagus to a level with the upper border of the thyroid cartilage, where it divides like the right common carotid into the external carotid and internal carotid.

Relations. The common carotid artery, in the neck, is inclosed in a fibrous

sheath, which also contains the internal jugular vein, lying to the outer side of the artery, and the pneumogastric nerve, which lies between and behind both. The sheath rests on the vertebral column, having interposed the sympathetic nerve and anterior muscles of the vertebral column, namely, longus colli and rectus anticus major; and being crossed behind by the inferior thyroid artery and recurrent laryngeal nerve. To the inner side of the carotid is the trachea, recurrent laryngeal nerve, and larynx; to its outer side, and inclosed in its sheath, the jugular vein and pneumogastric nerve; and in front, the sterno-thyroid, sterno-hyoid, sterno-mastoid, omo-hyoid, and platysma muscles, and the descendens noni nerve. The left common carotid, in addition to the relations just enumerated, which are common to both, is crossed near its commencement by the left vena innominata, and supports the remains of the thymus gland; it lies on the trachea; then gets to its side, and is in relation with the oesophagus and thoracic duct: to facilitate the study of these relations, I have arranged them in a tabular form.

Plan of the Relations of the Common Carotid Artery.

The External carotid artery (carotis externa) ascends nearly perpendicularly from opposite the upper border of the thyroid cartilage, to the space between the neck of the lower jaw and meatus auditorius, where it divides into two terminal branches, temporal and internal maxillary.

Relations. In front it is crossed by the posterior belly of the digastricus, stylo-hyoideus, and platysma myoides; by the hypoglossal nerve near its origin; higher up it is situated in the substance of the parotid gland, and has in front the facial nerve and temporo-maxillary vein. Behind, it is separated from the internal carotid by the stylo-pharyngeus and stylo-glossus muscle, glosso-pharyngeal nerve, and part of the parotid gland.