ANGEIOLOGY. 81 



plete rings. By the aid of the microscope, this coat may be divided into 

 three laminae: an external, thin, elastic, and yellow ; a middle, composed 

 of circular fibres similar to those of unstriped involuntary muscle ; and an 

 internal, similar to the last, but with the fibres longitudinal. The internal 

 coat of an artery is smooth and polished, and may be subdivided into two 

 laminffi, the internal composed of tesselated epithelium, and resting on a 

 basement membrane with longitudinal and internal fibres. Although the 

 external system is devoid of exact symmetry, yet, with a few exceptions, 

 one description will apply to either side. Arteries are all supplied with 

 nutrient vessels and nerves from the adjacent parts. ' 



A. The Arch of the Aorta. 



The aorta, arising from the superior posterior end of the left ventricle,' 

 passes beneath the pulmonary artery, and is entirely concealed in front by- 

 it. Keeping to the right, it emerges at the base of the heart, between the 

 right auricle and the trunk of the pulmonary artery, being bounded on the 

 right side by the descending vena cava. Continuing its ascent, it forms a 

 curvature with the convexity upwards, the summit of which arises to within 

 about an inch of the superior edge of the sternum. This curvature is in 

 front of the third and fourth dorsal vertebrge, and in its course the aorta 

 passes over the right pulmonary artery, across the left bronchus, and applies 

 itself to the left side of the spine about the third or fourth dorsal vertebra. 

 This bend is known as the aortic arch, arcics aortoi. In its descent down 

 the thorax the aorta is in contact with the left surface of the bodies of the 

 dorsal vertebrae. At the lower part of the thorax, it inclines towards the 

 middle line of the vertebra? in order to reach the hiatus aorticus of the 

 diaphragm, through which it penetrates to the abdomen. Here it descends 

 in front of the lumbar vertebrse, somewhat on their left side, ceasing at the 

 intervertebral space between the fourth and fifth vertebra? by division into 

 two large trunks, the primitive iliacs, one for each lower extremity and the 

 corresponding side of the pelvis. In the course of the aorta from the heart 

 to the loins, it first gives off the branches which supply the heart, then those 

 for the head and superior extremities, then those for the sides of the thorax, 

 and afterwards in the abdomen it detaches the trunks which supply the 

 viscera and sides of the abdomen. 



From the arch of the aorta there arise five arteries: the right and left 

 coronary, the innominata, the left carotid, and the left subclavian. Excep- 

 tions to this arrangement not unfrequently occur, as in addition to the two 

 coronary there are sometimes but two arteries, sometimes six. 



Coronary Arteries. The right and left coronary arteries are the 

 nutrient vessels of the heart. They arise above two of the sigmoid valves, 

 and communicate freely with each other by their ramifications. 



The arteria innominata, about an inch and a half in length, arises from the 

 upper part of the arch, at the junction between the ascending and horizontal 

 portion ; it ascends obliquely to the right side in front of the trachea and 

 of the right pleura, and opposite to the sternal end of the clavicle divides 

 into the right subclavian and right carotid arteries. 



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