A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element capable of changing length and rotating about an omnidirectional "hip" joint. As an anatomical animal structure it is used for locomotion. The distal end is often modified to distribute force (such as a foot). Most animals have an even number of legs. As a component of furniture, it is used for the economy of materials needed to provide the support for the useful surface such as the table top or chair seat. Terminology
Uniped: 1 leg, such as clams
Biped: 2 legs, such as humans and birds
Triped: 3 legs, which typically does not occur naturally in healthy animals
Quadruped: 4 legs, such as dogs and horsesMany taxa are characterized by the number of legs:

Tetrapods have four legs. Squamates of genus Bipes have only two. Caecilians and many squamate lineages convergently lost their legs. Panarthropoda: no less than 4 legs. Velvet worms and some arthropods have more than a dozen legs; a few species possess over 100. Despite what their names might suggest, centipedes ("hundred feet") may have fewer than 20 or more than 300 legs, and millipedes ("thousand feet") have fewer than 1,000 legs, but up to 750. Components
A leg is a structure of gross anatomy, meaning that it is large enough to be seen unaided. The components depend on the animal.
The smaller left arch passes anteriorly and to the left of the trachea in the usual position. It is often joined by the ligamentum arteriosum (or patent ductus arteriosus) where it forms the descending aorta. The left arch gives first origin to the left common carotid artery and then the left subclavian artery. The right aortic arch is completing the vascular ring by passing to the right and then behind the esophagus and trachea to join the usually left-sided descending aorta. The first vessel coming off the right arch is usually the right common carotid artery followed by the right subclavian artery. Double aortic arch with left dominant arch:
In this less common condition, as the name indicates, the left arch is the larger of the two aortic arches. The origins of the arm and head vessels are identical to the anatomy of double aortic arch with right dominant arch
Balanced or codominant double aortic arch:
In this rare condition both aortic arches are of the same diameter. Treatment
Surgical correction is indicated in all double aortic arch patients with obstructive symptoms (stridor, wheezing, pulmonary infections, poor feeding with choking). If symptoms are absent a conservative approach (watchful waiting) can be reasonable. Children with very mild symptoms may outgrow their symptoms but need regular follow-up. Anesthesia and intraoperative monitoring
The procedure is performed in general anesthesia. It is useful to place pulse oximeter probes on both hands and one foot so that test occlusion of one arch or its branches will allow confirmation of the anatomy.