A remarkable deviation from the cylindrical form, which is one of the general characteristics of the arterial system, is observable in two parts of the arch of the aorta; the first of these occurs at the commencement of this vessel in form of three dilatations corresponding to the semilunar flaps already described ; they were first pointed out by Valsalva, and have received the name of the lesser sinuses of the aorta ; they exist at all periods of life, and increase in size with years ; the other deviation from the cylindrical form is a dilatation on the right side of the ascending limb of the arch at its junction with the trans verse portion ; this dilatation, which does not exist in the foetus, grows larger as life advances, and appears to be produced by the impulse of the blood striking against this part of the aorta at each successive systole of the left ventricle. The aorta in the succeeding part of its course gradually grows smaller in- a degree` proportionate to the size of the branches it gives off.
The thickness of the aorta is proportionally less than that of its branches; it is thinner at its commencement than in the arch, in which part, according to Haller, it is thicker by an eighth on the convex than on the concave side; it gradually diminishes in thickness as it descends through the thorax and abdomen, but its power of resisting distention instead of being dimi nished in an equal degree was found by Win tringham to be greater at its lower part than near the heart.* The structure of the aorta is the same as that of the rest of the arterial system in general ; its external tunic, however, is slighter than that of all other arteries except those of the brain, it is weaker the nearer it is examined to the origin of the aorta ; it is strengthened near the heart by the covering which the serous layer of the pericardium gives to the aorta, and by an expansion from the fibrous layer of that mem brane, which is lost on the transverse portion of the arch. The cellular sheath of the aorta in which the soft fat around its origin is deposited, becomes so fine where the vessel is passing out of the pericardium as to lead some anatomists to deny its existence in this situation ; it becomes more evident in the course of the descending aorta through the mediastinum, and is still more considerable around the abdominal aorta, where it is usually loaded with a considerable quantity of adipose substance.
The branches which arise immediately from the aorta may be divided into orders, according to the degree of remoteness or the relative size and importance of the parts which they supply with blood ; first, the branches which convey blood to the two extremities of the trunk and the limbs attached to them ; these arteries, which are of considerable size, are the arteria innorninata, the Vic arotid and left subclavian, which, arising from the transverse portion of the arch, are distributed to the head, neck, and upper extremities, and the primitive iliac arte ries which arise from the lower part of the abdominal aorta supplying the pelvis and the lower extremities. 2nd order.—Branches some
what smaller going to the thoracic and abdomi nal viscera and the parietes of the chest and abdomen ; the coronary arteries which supply the heart arise from the aorta immediately after its origin ; the bronchial arteries which supply the substance of the lungs, and the intercostal arteries supplying the parietes of the chest arise from the thoracic aorta ; the coeliac, su perior and inferior mesenteric, which supply the digestive organs ; the renal arteries which supply the kidnies; the spermatic going to the organs of generation, the inferior phrenic sup plying the diaphragm, and the lumbar arteries going to the parietes of the abdomen and lum bar region of the spine, are the vessels of this order which arise from the abdominal portion of the aorta. 3rd order.—Branches of much smaller size are sent from the aorta to se condary parts which lie in its vicinity, as the thymus, the pericardium, the oesophagus, the renal capsules, ureters, &c. 4th order.—Small arterial twigs lost in the neighbouring cellular substance, lymphatic glands, and in the coats of the aorta itself.
Development. — The aorta appears to be formed in the fcetus prior to the heart and sub sequently to the system of porta, with which, to Baer, ltathke, and Meckel, it is connected by a small dilatation described by Dr. Allen Thomson* as a curved tube, which is the rudiment of the heart. (See Ovum.) Whilst the heart has but a single ventricle, the aorta and the pulmonary artery form a common trunk, which afterwards becomes divided by the de delopment of the contiguous portions of the circumference of both vessels ; during the remaining periods of intra-uterine life, and for a short time after birth, the pulmonary artery communicates with the aorta by the duc tus arteriosus, which appears as a continuation of the trunk of the pulmonary artery opening into the concavity of the arch of the aorta at its termination. The ductus arteriosus becomes impervious soon after birth, and having under gone a process of complete obliteration, is finally concerted into a ligamentous cord. The size of the arch of the aorta is less in proportion in the fcetus than in the adult, whilst the thoracic aorta is larger, being increased in size below the ductus arteriosus. The arch lies closer to the spine in the fcetus in consequence of the tra chea and bronchi behind it being so much less developed than in the adult, and the thymus which is it and the sternum being so much larger during life. In old age the curvature of the arch of the aorta is much greater in consequence of the great sinus having increased considerably in size.