Dorsal aorta

“ Trace the dorsal aorta backwards, and note that it gives rise to the cœliaco-mesenteric artery about midway between its origin and the origin of the first pair of renal arteries. The dorsal aorta then passes between the kidneys giving four renal arteries to each, and proceeds backwards and divides into an internal iliac artery to each leg. ”

“ In the Birds the right fourth arch alone remains as the aorta, the dorsal part of the left corresponding arch being obliterated. From the arch of the aorta rise two symmetrical innominates, each of which divides later into a carotid and subclavian. The blood path from the aorta to the hind limb in the Amphibia, Reptilia and Aves, is a dorsal one, and passes through the internal iliac and sciatic to the back of the thigh, and so to the popliteal space; the external iliac is, if it is developed at all, only a small branch to the pelvis. ”

“ Directly beneath the hindbrain is the notochord (nt) , on each side of which, in the mesoblast, is the dorsal aorta (ao) , or rather the continuation of the aorta into the head. Beneath these structures and extending from one side of the section to the other is the pharynx (ph) ; its lining wall is fused on each side with the ectoderm, but there is no actual opening to the exterior. These points of contact (g) between entoderm and ectoderm are of course the gill clefts; they are not yet visible from the outside. ”

“ Hence the pulmonary arteries yawn nearest for the blood, and, being short, wide vessels, present the least resistance to the first rush of blood-- mainly venous blood for the right auricle. As they fill up, the back resistance in them becomes equal, and then greater, than the resistance at A, and the rush of blood, now of a mixed quality passes through that aperture. It selects the dorsal aorta, because the carotid arch, plugged by the carotid gland, offers the greater resistance. ”

“ In horses a far larger proportion of aneurisms are deep-seated than in man, in whom forced muscular effort is less common. Yet even in horses the most common seat—the posterior aorta—is liable to overstretching and to inflammation and softening by reason of contiguity to dorsal sprains. The posterior 357aorta too, from its size and direction on leaving the heart, is in the direct line of the strongest blood current, and under long continued, forced and violent efforts (as in racing, hunting, and heavy uphill draughts) , has to sustain an extraordinary blood pressure. ”

“ The anastomosis of the intercostal arteries round the ribs is similar to the plexus, which is produced by the great sympathetic nerve in the same location. The lumbar arteries are given off farther posteriorly and along the lumbar region. The lumbar, like the dorsal branches are given off in pairs. These arterial branches pass along the thigh and the upper sacral vertebral region, and supply those parts. Ventrally they are covered by the kidneys and pass into the abdominal muscles. The posterior aorta gives off the spermatic arteries and the ovarian artery. ”

“ The umbilicus, A, indicates pretty generally the level at which the aorta bifurcates on the forepart of the lumbar vertebrae. In the erect, and even in the recumbent posture, the aorta may (especially in emaciated subjects) be felt pulsating under the pressure of the hand; for the vertebrae bear forward the vessel to a level nearly equal with, C C, the anterior superior spinous processes of the iliac bones. If a gunshot were to pass through the abdomen, transversely, from these points, and through B, it would penetrate the aorta at its bifurcation. ”

“ It is the summation of two factors largely; the force expended in opening the aortic valves (potential) and the force expended from that point to the end of systole, the force which is actually driving the blood to the periphery (kinetic) . To start the blood in motion, the heart must overcome a dead weight equal to the sum of all the forces [98] holding the aortic valves closed. This sum of factors, called the peripheral resistance, must be reached and passed by the force of the ventricular beat before one drop of blood is set in motion along the aorta. ”

“ In fact if they contracted like the heart to drive the blood, this fluid, tending as much to return towards the heart by the effect of this contraction, as to go to the extremities, there would be at intervals in the arterial tubes, valves to counteract this first motion;407 now we only see them at the origin of the aorta; why? because it is only necessary in the arteries to resist the effect of the contractility of texture, which, exerting itself without a jerk, by a mere contraction, can return but very little blood to the heart. ”

“ The aorta may or may not have plaques of nodular sclerosis, while the arteries, such as the radial or temporal, may be beaded or pipe stem in hardness. In spite of these far advanced peripheral lesions the aorta may appear smooth but it is markedly dilated, particularly the thoracic portion, it is noticeably thinned even on macroscopic examination, it has elongated as evidenced by its slight tortuosity, and it has lost the greater part of its elasticity. ”

“ The exaggerated pulsation sometimes observed in the abdominal aorta, the “pulsating aorta” seen in women, should not be mistaken for aneurysm.
Prognosis.—When natural cure occurs it is usually brought about by the formation of laminated clot, which gradually increases in amount till it fills the sac. Sometimes a portion of the clot in the sac is separated and becomes impacted as an embolus in the artery beyond, leading to thrombosis which first occludes the artery and then extends into the sac. ”

“ It occurs with greater frequency and with greater severity in men than in women.
Atheromatous degeneration is most common in the large arterial trunks, and the changes are most marked at the arch of the aorta, opposite the flexures of joints, at the mouths of large branches, and at parts where the vessel lies in contact with bone. The presence of diseased patches in the wall of an artery diminishes its elasticity and favours aneurysmal dilatation. Such a vessel also is liable to be ruptured by external violence and so give rise to traumatic aneurysm. ”

“ Aneurisms seated internally may occupy the cavity of the cranium, chest, or abdomen. As regards the first, little is known during life, for all the symptoms which they produce may arise from other causes. Aneurism of the anterior aorta may be situated very closely to the heart or in the arch, and it is very seldom that we can distinguish it from disease of the heart. The tumor may encroach upon the windpipe and produce difficulty in breathing, or it may produce pressure upon the vena cava or the thoracic duct, obstructing the flow of blood and lymph. ”

“ Branches for the head, neck and upper limbs; (2) branches for the viscera of the thorax and abdomen; (3) branches for the walls of the thorax and abdomen; (4) branches for the pelvis and lower limbs.
The branches for the head, neck and upper limbs arise as three large arteries from the transverse part of the aorta; they are named innominate, left common carotid and left subclavian. The innominate artery is the largest and passes upward and to the right, to the root of the neck, where it divides into the right common carotid and the right subclavian. ”

“ The caliber of the arteries does not dilate in old age. There is scarcely any except the arch of the aorta, which constantly undergoes an enlargement more or less considerable, which is always without rupture of the fibres, consequently supposes an extensibility of these fibres, and undoubtedly depends upon the habitual and direct impulse that the blood exerts against the concave side of this curvature. ”

“ Usually, at the beginning the symptoms are very obscure. A slight dyspnoea on exertion, or palpitation, or dyspeptic troubles are the chief complaint. The presence of these troubles in a man of fifty years or over, whose heart and kidneys present nothing abnormal, and in whom the smaller arteries of the extremities feel hard and calcareous, may excite the suspicion of atheroma of the aorta. There are no distinctive physical signs. Some writers speak of a short post-systolic murmur over the aorta beyond the valves, which may be audible only when the heart is acting strongly. ”

“ The most serious form of arterial thrombosis is that met with in the abdominal aorta, which is attended with violent pains in the lower limbs, rapidly followed by paralysis and arrest of the circulation. ”

“ If the carotid artery should tire out and not be able to perform its duty the brain would tire out also, and cease to operate. Should the descending aorta come to a halt from any cause, all parts of the body depending upon that vessel would suffer a total loss of blood supply. Equally so with any other principal artery of limb or body, all mark a failure equal to the suspended supply. The parts and principles of the human body depending upon the heart are numerous beyond computation. ”

“ Thus it is frequent in the posterior aorta towards its termination or in other words where the adjacent muscles (psoæ) are very liable to laceration from slipping backward or from efforts to disengage the limbs when fixed in soft ground; the femoral and auxiliary arteries are likewise frequent seats of inflammation and are likely to be overstretched when the limbs slip outwards.
Embolism or Plugging of the arteries must be accepted as another cause. ”

“ Again, one not infrequently encounters intimal nodosities when the underlying media appears of normal thickness. The explanation of this apparent exception is that the media in the living aorta is actually thinned, but the layers of subintimal tissue deposited over the weak spot due to strain hypertrophy become bulged inward when the pressure is relieved, as at postmortem. The media has not lost all of its elasticity (see Fig. 9) , hence it contracts and there is the appearance of a nodule on the intima beneath which is a media equal in thickness to that of the healthy surrounding media. ”