ABNORMALITIES OF ARTERIES. 897 
‘ Again, owing to epee 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 le ‘ft, a condition which is 
normal in birds. More rarely there are two permanent aortic arches, right and left, as in reptile She 
the cesophagus 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 aortee 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 aorte 
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 aortee 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 higher than the third or second 
lumbar vertebra. 
The aorta, instead of bifurcating into two common iliac arteries, may terminate in a common 
iliac artery on one side and an internal iliac artery on the opposite side, the external ilac 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 
biturcates into two internal iliac arteries, the external ilacs arising from the aorta at a higher 
level as lateral branches ; it is due either to a more extensive fusion than usual of the primitive 
dorsal aortee, or to the origin of the external iliac arteries from the primitive dorsal aorte being 
at a higher level than is ordinarily the case ; if the condition is due to the latter cause, it may 
be that the external iliac arteries in carnivores, and the external iliac arteries which oe casionally 
rise from the aorta in man, are somatic segmental arteries of a higher segment than the normal 
external iliac arteries of the human subject. 
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 sinus of Valsalva ; or again, their interventricular and transverse 
branches may all arise as distinct vessels from a single sinus of Valsalva. This variability i is not so 
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 of the innominate artery and of the roots of the subclavian arteries, to 
points beyond the origins of the vertebrals, into the arch. By variations of this process of 
absorption other combinations may be produced ; thus, instead of the roots of the subclavian 
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 
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 
number of branches then arising from the arch of the aorta is four, the additional vessel 
being the left vertebral, which arises between the left common carotid and the left subclavian. 
Occasionally the usual three branches from the arch are increased to four by the formation of a 
new vessel, the “ thyroidea ima.” This may be placed between the innominate and left carotid 
trunks, in which case it represents a persistent ventral visceral branch from the ventral root of the 
fourth left aortic arch ; in other cases the thyroidea ima springs from the innominate artery and 
represents a ventral visceral branch of the ventral root of the fourth right arch. Very-rarely 
the right vertebral artery arises separately, and forms a fourth branch of the arch of the aorta, 
the rest of the branches being normal. This condition cannot be accounted for by any modifica- 
tion of the ordinary developmental processes. It may possibly be due to the persistence of an 
irregular or unimportant anastomosis between the ventral root of an aortic arch and the seventh 
somatic segmental artery. 
Decrease in the number of branches from the arch of the aorta is most frequently due to 
fusion of the ventral roots of the fourth aortic arches, the result being that a stem is formed 
common. to the right subclavian and the right and lett common carotid arteries; whilst the 
left subclavian, arising separately, is the only other branch which springs from the arch of 
the aorta. 
If the fusion of the ventral roots proceeds further and includes those of the third arches, the 
result as regards the branches given off from the arch of the aorta is the same, viz. there is a com- 
mon stem for the right subelavian and both carotids, and a separate left subelavian trunk ; but 
1 It is to be observed that the exact point of bifurcation of the aorta, in relation to the vertebral 
column, is not entirely determined by the length of the descending aorta. 
57 
