It is composed of trapezius, latissimus dorsi, rhomboid major, rhomboid minor and levator scapulae. It is innervated by anterior rami of spinal nerves, reflecting its embryological origin outside the back. Intermediate group
The intermediate group is also known as respiratory group as it may serve a respiratory function. It is composed of serratus posterior superior and serratus posterior inferior. Like the superficial group, it is innervated by anterior rami of spinal nerves. Deep group
The deep group, also known as the intrinsic group due to its embryological origin in the back, can be further subdivided into four groups:

Spinotransversales - composed of splenius capitis and splenius cervicis. Erector spinae - composed of iliocostalis, longissismus and spinalis
Transversospinales - composed of semispinalis, multifidus and rotatores
Segmental muscles - composed of levatores costarum, interspinales and intertransversariiThe deep group is innervated by the posterior rami of spinal nerves. Organs near the back
The lungs are within the ribcage, and extend to the back of the ribcage making it possible for them to be listened into through the back. The kidneys are situated beneath the muscles in the area below the end of the ribcage, loosely connected to the peritoneum. A strike to the lower back can damage the kidneys of the person being hit. Surface of the back
The skin of the human back is thicker and has fewer nerve endings than the skin on any other part of the torso. With some notable exceptions (see, e.g. George "The Animal" Steele), it tends to have less hair than the chest on men.
Many English idioms mention the back, usually highlighting it as an area of vulnerability; one must "watch ones back", or one may end up "with ones back up against the wall"; worse yet, someone may "stab one in the back", but hopefully a friend "has got ones back". The back is also a symbol of strength and hard work, with those seeking physical labor looking for "strong backs", and workers being implored to "put their back into it". Historically, flagellation of a person across the back with a whip was both a common form of punishment of criminals, and a common means of forcing slaves to work. As well, self-flagellation, as in self punishment, may include the use of whipping oneself. This is one method of mortification, the practice of inflicting physical suffering on oneself with the religious belief that it will serve as penance for ones own sins or those of others. While more moderate forms of mortification are widely practiced—particularly in the Catholic Church—self flagellation is not encouraged by mainstream religions or religious leaders. A well-known instrument used for flagellations is the infamous Cat o Nine Tails, a nine-corded whip with one handle enabling a much more effective whipping than would be possible with only one lashing at a time. == References ==