264 CLINICAL APPLIED ANATOMY. 



immediately in front of or behind it will establish a communi- 

 cation between the left ventricle and the sinus portion of the right. 

 A defect in the fore part of the septum will establish a communi- 

 cation between the left ventricle and the infundibulum of the 

 right. 



Lesions at the origin of the aorta or in the aorta itself are 

 much less common than the corresponding lesions of the pul- 

 monary artery. Like the latter lesions they may involve the 

 conus arteriosus of the left ventricle, the orifice of the aorta or 

 the aortic trunk. Although rare, complete impermeability of 

 the aorta appears to occur more frequently than the correspond- 

 ing condition of the pulmonary stem. (Fig. 31.) 



Stenosis or atresia of the aorta may also occur at or below 

 the point where the ductus arteriosus joins it. Normally 

 the aorta shows a slight narrowing at the situation of the 

 ductus, to which the term aortic isthmus has been applied. 

 Stenosis or atresia at this spot probably arises in connexion 

 with the formation of the ductus. As a result of this 

 the descending aorta may be entirely supplied by the ductus 

 arteriosus. If the ductus becomes impervious or retains its 

 connexion with the proximal portion of the artery, a collateral 

 circulation is carried on by means of anastomoses between 

 branches of the subclavian arteries which arise above, and the 

 intercostal and phrenic arteries which arise below the constriction. 

 The presence of the enlarged anastomising arteries may enable 

 the diagnosis to be made during life. 



The aortic isthmus is succeeded by a fusiform dilatation known 

 as the aortic spindle. The lower limit of the spindle is supposed 

 to mark the site at which the atrophied right aortic arch joined 

 the descending aorta. A second constriction which sometimes 

 occurs lower than the constriction at the isthmus probably 

 arises in connexion with the obliteration of this right aortic 

 arch. 



Congenital atresia or stenosis of the auriculo-ventricular 

 orifices may be attributed to defects in the endocardial cushions 

 which subdivide the auricular canal. (Figs. 35 and 36.) 



