1 S84. 1 The Significance of Human Anomalies. 671 
the os centrale. This os centrale is always present in the 
higher apes and some of the rodents. We also find that in 
every human foetus, at an early period, a rudiment of this 
bone exists, but it has entirely disappeared by the fourth 
month of foetal life. 
Circulatory System . — Every naturalist now admits that the 
various stages of development of an animal, as well as its 
specialised parts, are often found to correspond with perma- 
nent conditions of animals lower in the scale. A good 
illustration of this is seen in the development of the human 
heart and blood-vessels. In the early stages of develop- 
ment we have a heart with a single cavity, connected with 
a vessel at each end, as in Ascidians ; later on the blood- 
vessels consist of a series of arches which go to the gills or 
branchial clefts, as in fishes and Amphibia, while the heart 
consists of two chambers separated by valves, and is placed 
far forward in the neck. The gill-arches now partly dis- 
appear, and, though the circulation still remains single, as 
in reptiles, the heart- cavities are beginning to be separated 
into two distinct systems. Soon a double circulation is 
acquired by a complete separation of the heart into right 
and left. The right heart propels the venous and the left 
the arterial blood. At this period the condition is identical 
with that of birds ; at last the true mammalian type of heart 
and blood-vessels developes, and remains permanent. The 
arrangement of the great blood-vessels going to and from 
the heart varies considerably in different mammals. In man 
the rule is for the great artery, carrying the blood from the 
heart to the general system, to give off three main branches, 
named the innominate, left carotid, and left subclavian. 
These are distributed to the head and the two arms ; the 
main vessel or aorta curves downward, and distributes blood 
to the trunk and lower extremities. These branches are now 
known to be derived from certain of the original gill-arches 
which persist, and when any variation in their arrangement 
takes place it always occurs in the line of some of these 
gill-arches ; that is, some of the arches persist which usually 
are obliterated. Nearly all the variations occurring in these 
large vessels in man are found to be the regular condition in 
animals lower in the scale ; for instance, sometimes only 
two branches are given off instead of three ; each of these, 
again, dividing into two, one for the head and one for the 
arm of that side. This is the usual arrangement in the 
bat, porpoise, and dolphin. The commonest variation of 
the aortic arch is where the innominate gives off the left 
