Circle of Willis or circulus arteriosus is formed at the interpenduncular space of the base of the brain by the union of the anterior cerebral arteries in front, posterior cerebral arteries behind and is completed laterally by the junction of the later with the posterior communicating arteries and by the internal carotid artery.
It is irregularly polygonal in outline and surrounds the optic chiasma and hypophvsis cerebri.
There is no rete mirabile cerebri in the horse.
Theintercostal arteries are 18 pairs in number.
The coeliac artery – It divides into
Left gastric
Hepatic
Splenic branches
The left gastric artery gives off pancreatic, esophageal, parietal and visceral branches.
The hepatic artery divides into pancreatic, right gastric and gastro duodenal branches.
The splenic artery is the largest of the three. It provided pancreatic, splenic, left gastroepiploic artery anastomoses with right gastroepiploic branch of hepatic artery after coursing along the greater curvature of the stomach.
Themiddle sacral artery is absent. However, the mediancoccygeal artery, an unpaired vessel originates from the posterior gluteal (right or left) artery takes the position of median sacral artery of ox.
The arteries and vascular arches of the limbs,
In the horse, the place of the collateral radial and collateral ulnar arteries is taken anterior radial and ulnar arteries, which arise as collateral branches of the brachial artery. The anterior radial artery supplies the extensors of the carpus and digit.
The ulnar artery anastomoses with the lateral volar metacarpal artery, one of the branches of the median artery under the cover of the flexor carpi ulnaris above the carpus to form supracarpal arch.
From this arch the lateral volar metacarpal artery descends downwards and below the carpus, it anastomoses with the medial volar metacarpal artery, a branch of the median to form the subcarpal or deep volar arch between the inferior check ligament and the suspensory ligament.
From this arch two vessels descend between the large and the small metacarpal bones on the posterior face and at the lower third they join together and this united vessel joins the large metacarpal artery or digital artery to form the distal volar arch.
The median artery continues as the large metacarpal artery or common digital artery, which divides into medial and lateral digital arteries. In the metacarpal region, it runs along the medial border of the flexor tendons and the vein in front and nerve behind.
Each digital artery descends parallel to the tendon of the deep flexor of the digit in company with the nerve and vein. At the fetlock, the artery is related to the vein in front and the posterior branch of the nerve behind. After detaching branches to the phalanges it enters the volar foramen of the third phalanx.
In the hind limb, posterior tibial artery is large and takes the place of saphenous artery in the lower third of the leg.
It divides into medial and lateral tarsal arteries.
The medial tarsal artery forms and S-shaped curve at the lower third of leg and divides into two plantar arteries. From the second part of the curve arises the recurrent tibial artery, which ascends and anastomoses with the posterior femoral artery. This anastomoses may be regarded as the supratarsal vascular arch. The formation of the subtarsal arch (proximate plantar by branches of the plantar and perforating tarsal) is the same as in ox).
From this arch, two superficialand deep plantar metatarsal arteries descend. The superficial metatarsal arteries are slender and appear to be the continuations of the planter arteries.
They join the corresponding small metatarsal bones and unite near the fetlock with the great metatarsal artery to form the distal plantar arch.
The great metatarsal artery is the direct continuation of the anterior tibial and passes obliquely on the anterior face of the large metatarsal bone and gains its posterior aspect and at the distal third divides into two digital arteries. The digital arteries resemble those in forelimb.
