The arms are joined to the spinal column (columna vertebrales) by the pectoral girdle. The proximal end of the humerus (humerus) is joined to the shoulder blade (scapula) in an articular joint and the distal end is joined to the two lower arm bones ulna and radius (ulna et radius), also in an articular joint.
The ulna and radius are joined together by an interosseous membrane (membrana interossea), which holds them together and transfers the load from one bone to the other. The lower arm muscles are also attached at this point.
In addition there are articular joints between the upper arm bone and the ulna (humero-ulnar joint), between the head of the radius and the upper arm bone (humeroradial joint), and between the head of the radius and the ulna (proximal radial-ulnar joint). At the distal end of the ulnar and radius there are articular joints to the carpal bones (ossa carpi).
An x-ray image of the elbow.
Please click on the image below for an animation on how the biceps works
The humerus (humerus)
In addition to the shaft (corpus), the humerus is also divided into a proximal (close to the body) and distal (away from the body) extremity. The proximal end is formed by the head of the humerus (caput humeri). This is followed directly by a recession, the collar of the humerus (collum anatomicum). At the front surface of the proximal end is a smaller muscular attachment tubercle (tuberculum minus), with a larger one at the back (tuberculum majus).
Between the two is a furrow (sulcus intertubercularis), which accommodates the tendon of the long head of the biceps. Two bone crests go in the distal direction from the two tubercules (crista tuberculi minoris et majoris). These are the attachment points for the major pectoral muscle and the two back muscles and the large round muscle. To the distal side of the two tubercles is the so-called surgical collar (collum chirurgicum). At around the middle of the shaft there is a rough area (tuberositas deltoidea) on the outside. This is the attachment point for the triangular deltoid muscle, which lifts the arm to the side (musculus deltoideus).
The shaft consists of a front medial surface (facies anterior medialis) with an edge pointing towards the ulna (margo medialis) and a front lateral surface (facies anterior lateralis) with a front lateral edge (margo lateralis). The sharp-edged ends (crista supracondylaris medialis et lateralis) provide the connection to the medial and lateral epicondyles (epicondylus medialis et lateralis) for the extensor and flexor muscles of the lower arm.
The distal end of the humerus (condylus humeri) consists of the trochlea (trochlea humeri) for the articulated joint with the ulna, and the head of the humerus (capitum humeri) for the articulated joint with the radius. A furrow (fossa olecrani) at the back above the trochlea is used to attach the elbow prominence (olecranon).
The fossa coronoidea proximal to the trochlea accommodates
the front of the two ulna processes (processus coronoideus) when
bent. Another furrow, fossa radialis, at the front above the
humerus bone, accommodates the head of the radius when severely
Please click on the image below for a 3D animation of the humerous.
Together with the radius (radius), the ulna makes up the lower arm bone. It has a shaft (corpus ulnae) and a distal (away from the body) and a proximal (close to the body) extremity.
At the proximal end of the ulna there is a strong hook-shaped process (olecranon) with a front articular surface (incisura trochlearis), which fits in the trochlea of the humerus (trochlea humeri). The incisura radialis is at the top on the side, this is an articular surface, which fits into the circumferentia articularis (surface on the head of the radius).
At the transition point to the ulna shaft there is a rough area (tuberositas ulnae) for attachment of the musculus brachialis, which bends the lower arm in the elbow joint. The three-sided surface of the ulna is separated by the laterally pointing interosseous edge (margo interosseus), an edge pointing to the front (margo anterior), and a rear edge (margo posterior).
The ulna head (caput ulnae) is at the distal end, surrounded by the circumferentia articularis, an articular surface at the side and front. Further in the distal aspect there is a small pin-shaped process, the processus styloideus.
At the proximal end of the radius, the radius head (caput radii) accommodates the head of the humerus in a furrow (fovea articularis) for the articular joint. This continues in the form of a rim-shaped surface (circumferentia articularis). Between the head of the radius and a medial rough area (tuberositas radii) for attachment of the biceps tendon, there is a narrower section, the collum radii.
When seen in cross section, the radius is almost three-sided in shape. The side pointing towards the radius (margo interosseus) is used to fasten the interosseous membrane (membrana interossea antebrachii), which offers attachment surface for other muscles and tendons. The other two edges are the margo posterior (rear edge) and margo anterior (front edge), which point forwards at the side. The attachment point for the pronator, the tuberositas pronatoria, is located in about the middle of the shaft surface.
At the distal end of the radius is the styloid process (processus
styloideus) with an insertion for the ulna (incisura ulnaris).
It is easily palpable from the outside. At the back there are
various pronounced furrows to accommodate the tendons of the
long extensor muscles. The tuberculum dorsale is easily palpable
here. At the distal end the articular surface facies articularis
carpi is used for attachment of the proximal hand joint.
Please click on the image below for a 3D animation of the radius.
The rear shoulder muscles raise and lower the pectoral girdle and turn and guide the arm backwards. The lower part of the trapezius muscle (musculus trapecius) holds the pectoral girdle when pushing up on the arms (for example on the parallel bars). Its middle part, which is the most powerful, holds the pectoral girdle under load, e.g. when carrying something.
Another rear shoulder muscle, the broad back muscle (musculus latissimus dorsi), turns and guides the arm backwards, for example when tying an apron behind the back. The lateral shoulder muscles are responsible among others for arm movements.
The serratus anterior muscle (musculus serratus anterior), for example, holds the pectoral girdle under resistance, e.g. during press-ups. It can also pull the shoulder blade forwards. This raises the shoulder joint upwards, the arm is lifted over the horizontal plane. The delta muscle (musculus deltoideus) is involved in all movements of the shoulder joint. It moves the arm backwards and forwards, and its middle part lifts the arm sideways to the horizontal plane.
An important muscle in the lateral pectoral girdle area is the large pectoral muscle (musculus pectoralis major). It allows for the arms to be rolled inwards and crossed. It moves the arms forwards, for example when throwing, swimming and boxing. Together with the broad back muscle, it pulls the arm down with great force, for example when chopping wood.
The supra- and infraspinatus muscles (musculus supraspinatus
and Musculus infraspinatus) work together with the teres minor
muscle (musculus teres minor) to make the upper arm roll
outwards, as necessary when writing.