220 The Anatomy of a 12-mm. Pig 
degenerating portion of the dorsal aorta extending to the aortic arch, 
beyond which it becomes a large trunk which unites with a similar ves- 
sel from the opposite side to form the median dorsal aorta. 
From the median aorta there arise pairs of intersegmental arteries. 
These formerly continued in single column along the right and left 
aorte, but after the development of the vertebral anastomosis they lose 
their aortic origin. In the 12-mm. pig the seventh intersegmental 
artery is the first to arise from the right aorta. From it the vertebral 
and subclavian arteries originate. 
The subsequent development of these vessels has been figured by 
Rathke, 43. The minute common carotid arteries become very long 
stems and fuse with one another below. Near their junction arises the 
right subclavian, of which the right aorta down to the 7th intersegmental 
artery forms a small section. Common carotids and right subclavian 
come from an innominate artery, not yet formed in the 12-mm. pig. 
The left subclavian leaves the aorta separately. Caudad from the 7th 
intersegmental artery, the right aorta is obliterated. 
The arteries of the brain have nearly their adult arrangement. The 
vertebrals unite in a long basilar artery with many lateral branches. 
In front of the pons it forks, forming the posterior communicating 
arteries, which pass into the internal carotids. A branch extending 
between optic stalks and brain forms the anterior cerebral artery which 
by anastomosing with the opposite side will complete the circle of 
Willis. In front of the mid-brain are the posterior cerebral arteries. 
The branches of the dorsal aorta are the regular pairs of interseg- 
mental arteries, the scattered mesonephric arteries and, in the median 
line, the coeliac axis and the omphalo-mesenteric artery. The last 
named formerly encircled the intestine (Hochstetter, 02, p. 114), but 
the left half of the loop has disappeared, and it now passes to the yolk 
sac on the right of the intestine. The large umbilical branches and 
the caudal extension of the aorta complete the arterial system. 
Veins.—The veins are shown chiefly in Plate 1V. The anterior cardi- 
nal system arises as a plexus between the developing hemispheres. The 
small vessels unite in a median stem, the anlage of the superior longi- 
tudinal sinus. This soon divides into two branches which pass around 
to the under side of the brain, thus forecasting the lateral sinuses. It re- 
ceives the ophthalmic vein, later subdivided into ophthalmic vein and 
cavernous sinus. Many branches from the mid-brain region unite with 
those just described and form the internal jugular vein. The 12-mm. 
pig agrees very closely with the 6-mm. guinea-pig studied by Salzer, 95, 
The internal jugular vein passes between the Gasserian ganglion and 
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—s- ee 
