| An arm is an upper limb of a human and some animals.
An upper limb is composed of the anatomic compartments listed below and of the corresponding bones:
Shoulder girdle (in informal literature and casual usage it is improperly called “shoulder”, including also the shoulder joint with the head of the humeral bone).
Clavicle;
Scapula;
Shoulder;
Humerus;
Forearm;
Elbow bone;
Radius;
Hand;
Wrist (8 bones located in 2 rows (numbered from a thumb))
The proximal row: navicular, lunate, triquetral, pisiform;
The distal row: trapeze, trapezoidal, capitate, hamate.
Metapodium;
5 bones, according to each of fingers, named by numbers, numbered from a thumb.
Fingers:
Each finger has three phalanxes (an exception is a thumb, having two phalanxes). The name of each of phalanxes originated from their location (proximal, medial, distal/nail) and from the number (or the name) of the finger (for example, medial phalanx of the second (forefinger) finger).
Also each human has so-called sesamoid bones; their location, sizes and quantity are extremely variable.
Muscles:
The muscular system of a hand consists of several layers of muscles, and many muscles are attached to more than one joint, thanks to that, in case one of the muscles is activated a corner in several joints changes as well.
Innervation:
A hand has efferent and afferent innervations. Efferent fibers send signals from a spinal cord to a hand and afferent fibers — from a hand to a spinal cord (through dorsal ganglions). Fibers are split in nerves, and practically all of them are mixed, it means they contain both efferent and afferent fibers.
Receptors of the skin, muscles and joints:
A hand is provided with a large quantity of the sensory endings (when hands "grow numb", it is a sign that something is wrong with them).
How the brain operates a hand:
The cerebral cortex has sections responsible for the management of certain parts of the body. The sections are often represented as homunculus — a little human flatting on the cortex. His feet are dorsally located, that is closer to the vertex, at the same time arms and the face are ventrally located, and that is on one side of the head, though a body is followed by an arm, which is followed by a face. This homunculus has thumbs and a palm and rather small forearm and a shoulder. Actually, there is not only one but many homunculus in the cortex, as for practically arms and the head, as well as feet are presented in each specific area. For example, a part of the cerebral cortex, called the primary motor cortex, has a section, which is activated each time when a human makes a movement by an arm, and the primary somatosensory cortex has a section, which is activated when a human touches some object by a hand.
Except cortical sections, the centers responsible for management of an arm are in a cerebellum and its nucleus, thalamus, bazal ganglions, a brain axis and a spinal cord. This complex network of the neurons carries out wide range of movements of an arm; though, automatic movements (for example, movements to keep balance) should be distinguished from voluntary ones (threading a needle). The cortex of cerebrum is responsible for complex voluntary movements, and the centers of the lower level - for automatic.
Electric stimulation of the brain enables movements of the fingers and of the whole arm. During surgery it is usually done by means of electrodes attached to the brain surface, or settled directly into the brain. It is also possible to stimulate the brain via the skin and cranial bones. A focal magnetic stimulation is used for this purpose. |
The classic theory of evolution tells us that Neanderthal men had very long arms which they inherited from monkeys. The long arms allowed to masterfully climb the trees and swing from one branch to another, they were used as an additional support in walking, and they allowed to get an edge in close fights with predators.
