CHAPTER XII. 

 THE SPINAL CORD. 



The spinal cord is an organ composed largely of 

 neurons, with which are associated blood-vessels, 

 connective- tissue elements, and a limited amount of 

 epithelial and muscle cells. It represents the ter- 

 minal portion of the cerebrospinal axis and is a 

 direct continuation of the encephalon. It is one of 

 the first organs to develop in the embryo where it 

 makes its appearance as a dorsal ectodermal groove. 

 This neural groove gradually closes to form a canal 

 which lies at first just beneath the ectoderm and 

 later becomes encased in connective-tissue layers and 

 the bony axial skeleton. 



The cord is bilaterally symmetrical and flattened 

 dorso-ventrally. It presents two enlargements: 



(1) the upper or cervical, which extends from the 

 third cervical vertebra to the second thoracic and 

 corresponds to the origin of the nerves of the arm, and 



(2) the lower or lumbar, which extends from the 

 ninth thoracic vertebra to the terminal cone at the 

 level of the body of the second lumbar vertebra. 

 The lumbar enlargement marks the origin of the 

 nerves of the leg. From the apex of the terminal 

 cone there extends a slender rudimentary prolonga- 

 tion, the filum terminale, which, with the spinal 

 nerves of this region, is called the cauda equina. The 



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