This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The spinal cord in the male is 45 cm. (18 in.) long. In the foetus of three months the cord extends to the end of the spinal canal; at birth it has risen to the third lumbar vertebra, and in the adult it is opposite the lower border of the first. It will thus be seen that the point of exit of the spinal nerves from the cord is always some distance higher up than their exit from the intervertebral foramina. The cord is enlarged in the cervical and lumbar regions, the cervical enlargement ending opposite the second dorsal, and the lumbar enlargement beginning about opposite the tenth dorsal vertebra and decreasing gradually. These enlargements correspond with the origin of the nerves to the upper and lower extremities.
The spinal cord is divided into so-called segments. These are thirty-one in number; eight are cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal. Each segment embraces that portion of the cord which gives exit to one pair of anterior or motor root fibres and receives one pair of posterior or sensory root fibres. These segments are a variable distance above the point of exit of the nerves from the bony spinal column. Besides motor and sensory functions they exercise control over certain reflex movements and functions. The control of the bladder and rectum is located in the fifth and sixth sacral segments; the cremasteric reflex is governed by the first and second lumbar; the plantar or Babinski reflex by the first to third sacral, as is also ankle clonus; and the patella reflex by the second and third lumbar segments. They likewise exert a trophic influence, and the appearing of bed or pressure sores without ample cause is presumptive evidence of a spinal lesion of the segments supplying the part.
Fig. 485. - Showing the method of articulation of the eleventh and twelfth dorsal and the first and second lumbar vertebrae.