CHAPTER VI. 

 THE SPINAL CORD. 



The spinal cord (medulla spinalis) is developed from the posterior 

 part of the neural tube, and forms the corresponding portion of 

 the central axis of the nervous system. 



Extent. It is continuous with the medulla oblongata, above; 

 and, in the adult, reaches to the lower border of the first lumbar 

 vertebra (Fig. 100). Its length is seventeen to eighteen inches. 

 In a very slender process, the filum terminate internum, the cord 

 is continued beyond the first lumbar vertebra. That process 

 and the lower spinal nerves form the cauda equina, which is in- 

 closed in a sheath composed of the arachnoid and dura mater. 

 The filum terminale internum for about three inches contains a 

 prolongation of the central gray matter and ventricle of the cord; 

 and, also, a few fibers, which suggest the coccygeal nerves of lower 

 animals. 



In the foetus before the third month, the cord and spinal canal 

 are of equal length. At birth the cord reaches the third lumbar 

 vertebra, and it continues to recede with the rapid growth of 

 the vertebras to adult life. 



Diameters (Fig. 101). The spinal cord is shaped like a cylin- 

 der, slightly flattened from before backward (dorso-ventrally). Its 

 longest diameter is transverse and measures less than half an inch, 

 except hi the cervical and lumbar enlargements of the cord. 

 In the latter, it equals a half -inch; and, hi the former, it slightly 

 exceeds it. The thoracic portion of the cord is small and nearly 

 cylindrical in shape. Divested of its meninges and nerves the 

 spinal cord weighs about one ounce and a half, avoirdupois. 



The cervical enlargement (intumescentia cervicalis) extends 

 from the medulla oblongata to the second thoracic vertebra 

 (Figs. 100 and 101). Its greatest diameter is on a level with the 

 fifth intervertebral disk. It gives origin to the motor fibers and 



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