SPINAL CORD. 



105 



tern contained within the spinal canal. It 

 connects with the brain through the medulla 

 oblongata, and terminates in a fine thread of 

 gray matter (the filum terminale) at about 

 the second lumbar vertebra. In form it is 

 irregularly cylindrical, and varies in the size 

 and shape of its cross-section at various lev- 

 els, as is shown in Fig. 27. It is incom- 

 pletely divided into symmetrical halves, and 

 its mid-line is indicated in front by a fissure 

 (anterior median fissure) which extends for 

 about one-third its antero-posterior diameter ; 

 behind by a deeper but narrower fissure (pos- 

 terior median fissure), which involves about 

 one-half of the same diameter. It is com- 

 posed of white and gray substance. 



How are the white and gray matters ar- 

 ranged in the spinal cord? 

 The white substance is arranged externally 

 to the gray in each half of the cord, and is 

 so disposed as to be conveniently divided for 

 purposes of description into three columns, 

 known respectively as the anterior, lateral, 

 and posterior columns of the cord. There is 

 also a thin band of white substance at the 

 base of the anterior median fissure (the white 

 commissure). The gray matter fills in the 

 central portion of the cord, and is variable in 

 its amount, the calibre of the cord at its en- 

 largements being increased by the increase 

 in the amount of gray matter at these points 

 (Fig. 27). The white substance will be no- 

 ticed to diminish quite regularly in the sec- 

 tions of the cord from above downward, as 

 seen in this series. The gray substance is not 

 completely halved by the anterior and pos- Transverse Sections of the 



. * _ ^ r> t t i Sninal PnrH in Man T 



tenor fissures of the cord, but is continuous 

 across the mid-line ; and in it at the centre 

 is a minute canal communicating with the 

 ventricles of the brain. The gray matter is 

 more abundant between the lateral and anterior and between the 



Spinal Cord in Man : I, 

 upper cervical region ; 



II, lower cervical region ; 



III, dorsal region ; IV, 

 lumbar enlargement ; V, 

 lower extremity. 



