SPINAL CORD. 



63 



The posterior horn consists of two parts, the caput cornu, or expanded ex- 

 tremity of the horn (Fig. 30), round which is the lighter space or lamina, the 

 gelatinous substance; and the cervix cornu, or remaining narrow portion of the 

 horn, as far forwards as the central canal. 



The gelatinous substance contains along its border a series of large nerve- 

 cells ; but more internally consists of a stratum of small cells traversed by 

 transverse, oblique, and longitudinal fibres (Figs. 31, 32). 



Fig. 31. 



Transverse section of the gray substance of the spinal cord through the middle of the lumber enlargement. 

 On the left side the groups of large cells are seen ; on the right side the course of the fibres without the cells. 

 (Magnified 13 diameters.) 



Nearly the whole inner half of the cervix is occupied by a remarkable and 

 important column of nerve-cells, called the posterior vesicular column (Fig. 30), 

 which varies in size and appearance in different regions of the cord, and is 

 intimately connected with the posterior roots of the nerves. 



Within, and along the outer border of the cervix, are several thick bundles 

 of longitudinal fibres, represented in Fig. 30 by the dark spots ; other bundles 

 of the same kind may be seen in the gray substance along the line of junction 

 of the caput with the cervix cornu (Fig. 31). 



The anterior horn of the gray substance in the cervical and lumbar swellings, 

 where it gives origin to the nerves of the extremities, is much larger than in 

 any other region, and contains several distinct groups of large and variously 

 shaped cells. This is well shown on comparing the above figures. 



The tractus intermedio-lateralis (Fig. 30) extends from the upper part of the 

 lumbar to the lower part of the cervical enlargement, and consists of variously 

 shaped cells, which are smaller than those of the anterior cornu. In the neck 

 above the cervical enlargement, a similar tract reappears, and is traversed by 

 the lower part of the spinal accessory nerve. 



Origin of the Spinal Nerves in the Cord, The posterior roots are larger than the 

 anterior ; but their component filaments are finer and more delicate. They are 

 all attached immediately to the posterior columns only, and decussate each 

 other in all directions through the columns ; but some of them pass through 



