AORTA. 



189 



the chest well formed, and the heart and the 

 arch of the aorta free from disease, the origin 

 of the aorta is opposite the sternal articulation 

 of the cartilage of the fourth rib of the left 

 side in the male, and the intercostal space 

 above it in the female ; the ascending lirnb of 

 the arch, which is behind the middle bone 

 of the sternum in the greater part of its length, 

 may be felt pulsating on the right side of the 

 sternum in the second intercostal space ; the 

 highest part of the transverse portion of the 

 arch is on a plane with the centre of the sternal 

 extremities of the first pair of ribs, and about 

 an inch below the upper margin of the ster- 

 num : the arch of the aorta terminates oppo- 

 site the lower edge of the cartilage of the 

 second rib of the left side. 



The thoracic aorta descends in the posterior 

 mediastinum, and advances from the left side 

 to the front of the thoracic portion of the spine, 

 crossing in its course the left intercostal veins, 

 and the left vena azygos when that vein exists ; in 

 front it is covered by the left bronchus, the pos- 

 terior surface of the pericardium, the lower ex- 

 tremity of the oesophagus, and the left stomachic 

 cord of the par vagum ; on the right side it is 

 bounded by the oesophagus, thoracic duct, and 

 vena azygos ; on the left side it is covered by 

 the pleura, and in contact with the internal 

 surface of the left lung, and at its lower extremity 

 the left splanchnic nerve comes into contact 

 with it, and most frequently accompanies it 

 through the diaphragm. 



The abdominal aorta, which enters the abdo- 

 men between the crura of the diaphragm, des- 

 cends along the front of the abdominal ver- 

 tebrae and the left lumbar veins ; it is covered 

 in front by the solar plexus of nerves, the 

 stomach, pancreas, transverse portion of the 

 duodenum, the splenic and left renal veins, the 

 small intestine, and the root of the mesentery ; 

 on the right side it is bounded by the abdomi- 

 nal vena cava, and the commencement of the 

 thoracic duct, and on the left it is covered by 

 the peritoneum going to form the left layer 

 of the mesentery. The termination of the aorta 

 in the common iliacs and the middle sacral 

 arteries is a little below the level of the um- 

 bilicus. 



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 

 innominata, the leftc arotid and left subclavian y 

 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 cceliac, 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- 



* Experimental Inquiry on some parts of the 

 Animal Structure. London, 1740. 



