The nervous system preserves more than any other part of the human body the original division of the embryonic tube into metameres. The spinal cord is divided in segments, each segment being provided with a pair of anterior and posterior roots. The area of skin which is supplied by the fibres of a certain spinal root is called a dermatome. The skin is innervated by the posterior roots as well as by the anterior.
The former carry the afferent sensory fibres and efferent fibres subserving vasodilatation. The anterior roots carry efferent motor, sudorific, pilomotor and vasoconstrictor fibres, and also afferent sensory fibres. In this lecture I shall deal exclusively with the fibres passing by the posterior roots.