The Vascular System 213 
arches arises the most anterior of seven or eight 
pairs of lumbar 
arteries, lu I-7; 
this first lumbar 
artery is contin- 
ued cephalad for 
some distance as 
a longitudinal 
trunk that gives 
off severallateral 
branches to the 
walls of the thor- 
acicregion. The 
other six or seven 
lumbars are dis- 
tributed to the 
dorsal body wall, 
and arise, at 
more or less reg- 
ular intervals, as 
far caudad as the 
sacrum, or even 
back of that 
point. 
The first large 
branch of the 
aorta is the un- 
paired mesen- 
Fic. 61. The arteries of the posterior 
region of the Florida alligator.—For lettering, 
see pages 224-25. 
teric artery, m', which is given off in about 
the region of the fourth pair of lumbars; it 
