No. 2.] VENTRAL ABDOMINAL WALLS IN MAN. 365 
Hochstetter has settled definitely that the subclavian is a 
branch from the seventh cervical segmental artery. Between 
the seventh cervical (if we number the arteries with the nerves) 
and the third dorsal we have three segmental arteries. In Fig. 
15 the segmental arteries in the region are all shown to be 
simple with the exception of the seventh, which sends a marked 
branch into the arm. From this stage to the one pictured in 
Fig. 16 there is a marked jump, but in it we see the intermediate 
stage between Fig. 15 and the adult. 
All of the arteries below the vertebral are destined to go 
behind the sympathetic, and it is only excluded on account 
of its direction. In embryo XLIII (Fig. 16) the eighth cervical 
segmental passes on the ventral side of the nerve, which shows 
conclusively that it must either be a new artery or a secondary 
connection between the eighth segmental and subclavian. Since 
the first and second intercostal arteries pass behind the sympa- 
thetic in this embryo and in front of it in the adult, we must 
accept Hochstetter’s opinion that the superior intercostal is 
formed by secondary connections between the upper intercostals 
and the subclavian. If the old connection remains, it forms the 
arteria aberrans. 
I stated above that the sympathetic lies in front of the 
subclavian, while in the adult it lies in great part behind it. 
Hochstetter explains this change of position by a wandering of 
the trunk of the artery through the group of embryonic nerve 
cells. In the early embryos the sympathetic system is as a 
group of sprouts from the segmental nerves which cross the 
segmental arteries, and the sympathetic cord is of secondary 
formation. This cord grows very rapidly during the fifth and 
sixth weeks, and makes a great mass of cells extending from 
the vagus ganglion to the adrenals, connecting all of the 
branches with the segmental nerves to make of them rami 
communicantes. This all goes on hand in hand with the 
descent of the heart into the thorax. At the same time the arm 
is rotating towards the ventral median line, and drags with it 
1 Hochstetter numbers the segmental arteries to correspond with the vertebrae 
above them. Throughout I give the arteries the numbers of their accompanying 
nerves. He states that the subclavian arises from the sixth segmental, this being 
what I call the seventh cervical segmental. 
