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HANDBOOK OF PHYSIOLOGY. 



canal), which is continued through the whole length of the cord, and 

 opens above into the space at the back of medulla oblongata and pons 

 Varolii, called the fourth ventricle. It is lined by a layer of columnar 

 ciliated epithelium. 



The spinal cord consists of two exactly 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, in front of the central canal, con- 

 sisting of medullated nerve-fibres, and a posterior commissure behind 

 the central canal, consisting also of medullated nerve-fibres, but with 



A , 



FIG. 830. Different views of a portion of the spinal cord from tin cervical region, with the 

 roots of the nerves (slightly enlarged). In A, the anterior surface of the specimen is shown; the 

 anterior nerve- root of its right side being divided; in B, a view of the right side is given; in c, the 

 upper surface is shown; in D, the nerve-roots and ganglion are shown from below. 1. The anterior 

 median fissure; 2, posterior median fissure; 3. anterior lateral depression, over which the anterior 

 nerve-roots are seen to spread; 4, posterior lateral groove, into which the posterior roots are seen 

 to sink; 5, anterior roots passing the ganglion; 5', in A. the anterior root divided; 6, the posterior 

 roots, the fibres of which pass into the ganglion 6'; 7, the united or compound nerve; 7', the poste- 

 rior primary branch, seen in A and u to be derived in part from the anterior and in part from the 

 posterior root. (Allen Thomson.) 



more neuroglia, which gives the gray aspect to this commissure (Fig. 

 330, B). Each half of the spinal cord is marked on the sides (obscurely 

 at the lower part, but distinctly above) by two longitudinal furrows, 

 which divide it into three portions, columns, or tracts, an anterior, 

 lateral, and posterior. From the groove between the anterior and lateral 

 columns spring the anterior roots of the spinal nerves (B and 0, 5); and 

 just in front of the groove between the lateral and posterior column arise 



