FIG. 61. The arteries of the posterior region of the Florida alligator.--For lettering, see pages 224-25.] The first large branch of the aorta is the unpaired _mesenteric_ artery, m¹, which is given off in about the region of the fourth pair of lumbars; it carries blood through the mesentery to the greater part of the small intestine and also sends a small branch to the large intestine. Posterior to the mesenteric, the aorta gives off four or five pairs of short arteries, the _urogenitals_, _u 1-4_, that lead to the nearby reproductive organs and kidneys. About the middle region of the kidneys, a short distance anterior to the sacrum, is given off a pair of rather large arteries, called by Bronn the _ischiadicæ_, is¹; each ischiadica, after giving off a couple of small branches to the back, passes laterad and divides into three main branches: (1¹) to the ventral body wall, (3¹) to the anterior border and deeper region of the thigh, and (2¹) to the pelvis. In the region of the sacrum is given off a pair of _iliac_ arteries, il¹. Each iliac is of about the same diameter as the ischiadica and gives off, soon after leaving the aorta, an artery, ab, that apparently leads chiefly to the abdominal muscles. Distal to the origin of the abdominal, the iliac gives off a small _pelvic_ artery, pa, which leads, as the name would indicate, to the pelvis. The iliac then passes into the thigh, where it gives off several large branches and may be called the _sciatic_, sc. At the knee the sciatic gives off two rather small branches: one, the _fibular_ artery, f¹, extends down along the posterior side of the lower leg; the other is parallel to the first and may be called the _tibial_ artery, tb, since it extends along the anterior or tibial side of the shank. These two arteries give off numerous branches to the muscles of the lower leg. After giving off the fibular and tibial arteries, the sciatic passes, as a large vessel, through the lower leg, to which it gives but few branches, and may here be called the _crural_ artery, cr. At the tarsus it divides rather suddenly and, perhaps, variably, into four chief branches, leading to the toes. [Illustration: FIG. 62. The arteries of the anterior region of the Florida alligator. The arteries of the left foreleg shown at A.--For lettering, see pages 224-25.] A short distance caudad to the origin of the iliacs the dorsal aorta gives off a pair of small _pelvic_ arteries, pa¹, going to the muscles of that region. Caudal to these pelvic arteries is given off the unpaired _first hæmorrhoidal artery_, he¹, which divides into a _rectal_, rt¹, and a _cloacal_, cl, branch. Caudal to the first hæmorrhoidal arises the _second hæmorrhoidal_, he²; also unpaired, leading to the cloaca. Posterior to the second hæmorrhoidal, the aorta continues into the tail as the large _caudal_ artery, ca.