CHAPTEE XLV 



STRUCTURE OF THE SPINAL CORD 



THE spinal cord is a column of nerve-substance connected above with 

 the brain through the medium of the bulb, and situated in the spinal 

 canal. In transverse section it is approximately circular, but the 

 cord is not of the same size throughout its course. It exhibits two 

 enlargements, one in the cervical, the other in the lumbar region. 

 These are the situations whence the large nerves for the supply of 

 the limbs issue. The cord ends below, about the lower border of the 

 first lumbar vertebra, in a cone-shaped termination (the conus 

 medullaris) from which passes a slender filament of grey substance, 

 the filum terminate, which lies in the midst of the roots of many 

 nerves forming the cauda equina. 



It is composed of grey and white matter ; the white matter is 

 situated externally, and constitutes its chief portion ; the grey matter 

 is in the interior, and is so arranged that in a transverse section of 

 the cord it appears like two crescentic masses (the horns of each of 

 which are called respectively the anterior and posterior cornua) con- 

 nected together by a narrower portion or isthmus, called the posterior 

 commissure (fig. 399). Passing through the centre of this isthmus 

 in a longitudinal direction is a minute canal ; in a transverse section 

 it appears as a hole ; this central canal of the spinal cord is continued 

 throughout its entire length, and opens above into the space at the 

 back of the medulla oblongata and pons Varolii, called the fourth 

 ventricle. 



The spinal cord consists of two symmetrical halves, separated 

 anteriorly and posteriorly by vertical fissures (the posterior fissure 

 being deeper, but less wide and distinct than the anterior), and 

 united in the middle by nervous matter which is usually described 

 as forming two commissures an anterior commissure (consisting of 

 medullated nerve-fibres) in front of the posterior commissure, which 

 is the isthmus of grey matter pierced by the central canal, to which 

 we referred in the last paragraph (fig. 399, B). Each half of the spinal 

 cord is marked on the sides (obscurely at the lower part, but dis- 



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