ABNOEMALITIES OF AETEEIES. 1051 



ABNOEMALITIES OF THE HEAET. 



The heart may be transposed from the left to the right side of the body, a condition which 

 is usually associated with general transposition of the viscera, and with the presence of a right 

 instead of a left aortic arch. 



The external form of the heart does not as a rule vary much, but occasionally the apex is 

 slightly bifid, a character it normally possesses at an early stage of its development, and which is 

 retained in the adult in many cetaceans and sirenians. The internal conformation of the heart 

 deviates from the normal much more frequently ; more particularly is this the case with regard 

 to the septa which separate the right from the left chambers. The interatrial septum may be 

 entirely absent, as in fishes ; it may be fenestrated and incomplete, as in some amphibians ; or 

 the foramen ovale may remain patent, as in amphibians and reptiles. 



The interventricular septum may be absent, as in fishes and amphibians, or incomplete, as in 

 reptiles ; when incomplete, it is usually the " pars membranacea septi " which is deficient, but 

 perforations are occasionally found in the muscular portion. 



The communication between the infundibular part of the right ventricle and the body of the 

 ventricle may be constricted or the infundibular part may be entirely cut off from the remainder 

 of the cavity. 



ABNOEMALITIES OF AETEEIES. 



The pulmonary artery and the aorta may arise by a common stem, as in fishes and some 

 amphibians, and the common stem may spring either from the right or the left ventricle, or from 

 both. In these cases the truncus arteriosus has remained undivided, and the normal position of 

 the interventricular septum in relation to the lower orifice of the aortic bulb has been altered. 



Again, owing to malposition of the aortic septum, the pulmonary artery may spring from the 

 left ventricle and the aorta from the right ventricle. . In some cases the root of the pulmonary 

 artery is obliterated, and the blood passes to the lungs along the patent ductus arteriosus. 



Occasionally the arch of the aorta is on the right side instead of the left, a condition which is 

 normal in birds. More rarely there are two permanent aortic arches, right and left, as in reptiles ; 

 the oesophagus and trachea in these cases are enclosed in a vascular collar, the two arches unite 

 dorsally, and the beginning of the descending aorta is double. Quite independent of this condi- 

 tion, however, the two primitive dorsal aortse sometimes fail, either altogether or partially, to 

 unite together, and the descending aorta is accordingly represented, to a corresponding extent, by 

 two tubes. A more common, though still rare, form of double aorta is that due to the persistence, 

 in whole or in part, of the septum formed by the fused walls of the primitive dorsal aortae 

 from which the descending aorta is developed. 



The length of the descending aorta is determined largely by the extent to which fusion of the 

 two primitive aortse takes place. Accordingly, when this deviates from the normal, the termi- 

 nation of the descending aorta is at a correspondingly higher or lower level than usual, and 

 resulting from this the lengths of the common iliac arteries are almost invariably proportionately 

 modified. The bifurcation of the aorta may be as low as the fifth lumbar vertebra ; less 

 frequently it is higher than usual ; it is rare, however, to find it as high as the third, and still 

 more rare to find it at the level of the second, lumbar vertebra. 



The aorta, instead of bifurcating into two common iliac arteries, may terminate in a common 

 iliac artery on one side and a hypogastric artery on the opposite side, the external iliac artery 

 on the irregular side arising, at a higher level, as a branch of the aortic stem. This arrangement 

 approaches the condition met with in carnivores and many other mammals, in which the aorta 

 bifurcates into two hypogastric arteries, the external iliacs arising from the aorta at a higher 

 level as lateral branches ; it is probably due either to a fusion of the secondary roots of the 

 umbilical arteries of opposite sides. 



THE BRANCHES OF THE AORTA. 



The coronary or cardiac arteries may arise by a single stem. When arising separately 

 both may spring from the same aortic sinus ; or again, their interventricular and circumflex 

 branches may arise as distinct vessels from a single aortic sinus. This variability is not 

 very remarkable, seeing that the arteries in question are merely enlarged " vasa vasorum " 

 raised to a position of special importance by the development of the heart. 



The branches of the arch of the aorta are sometimes increased and sometimes decreased in 

 number. 



The highest number recorded is six, viz., right subclavian, right vertebral, right common 



( carotid, left common carotid, left vertebral, and left subclavian. Apparently this condition is the 

 result of the absorption into the arch of the innominate artery and of the roots of the sub- 

 clavian arteries, to points beyond the origins of the vertebrals. By variations of this process of 

 absorption other combinations may be produced ; thus, instead of the roots of the subclavian 



i arteries being absorbed, the right common carotid and innominate arteries may alone be absorbed, 

 in which case the five following branches spring separately from the arch of the aorta : right 



i subclavian, right external carotid, right internal carotid, left common carotid, and left 

 subclavian. The trunk most commonly absorbed is the initial part of the left subclavian ; the 



