THK NERVOUS SVSTKM 4G1 



Variations in Structure at Different Levels 



While the general structure above described obtains throughout 

 the cord, the size and shape of the cord, the size and shape of the 

 gray matter, and the relative proportion of gray matter and white 

 matter, vary in different parts of the cord, which must therefore be 

 separately considered. These variations are due to: (i) Variations 

 in the size of the nerves entering and leaving, which cause correspond- 

 ing variations in the gray matter which receives the afferent fibres 

 and contains the cells of origin of the efferent fibres. Thus the 

 larger nerves of the extremities cause the increase in size of the gray 

 matter of the cervical and lumbo-sacral cord with which they are con- 

 nected, and also an increase in the dorsal funiculi. (2) A gradual 

 increase in the white matter of the cord, as higher levels are reached, 

 due to an increase in the number of long ascending and descending 

 fibres to and from the brain. 



PRACTICAL STUDY 



Section through the Twelfth Thoracic Segment (Fig. 324). — Note that the 

 cord is smaller than in the lumbar enlargement and somewhat flattened dorso- 

 ventrally; that the amount of gray matter and white matter is diminished; that 

 both anterior and posterior horns are more slender, the anterior horn containing 

 comparatively few cells. At the inner side and base of the posterior horn may be 

 seen the group of cells known as Clarke's column (p. 456). MeduUated fibres 

 can be seen passing from the dorsal funiculus into Clarke's column, where they 

 interlace among the nerve cells. These fibres are collaterals of the dorsal root 

 fibres terminating in the nucleus. From the nucleus coarser fibres can be seen 

 gathering at its ventral side and thence passing outward to the periphery of the 

 cord where they bend upward forming the beginning of the dorsal spino-cerebellar 

 tract (see p. 469). 



Section through the Mid-thoracic Region (Fig. 324). — Compare with the 

 lumbar sections. Note the change in shape and size; that the cord is more 

 nearly round and smaller; that while the reduction in size affects both gray matter 

 and white matter it is the former that shows the greater decrease. The horns 

 are even more slender than in the twelfth thoracic section, and the anterior 

 horn contains stUl fewer cells. Clarke's column is present, but not so large. 



Section through the Cervical Enlargement (Fig. 323). — Note the marked in- 

 crease in size of the cord, which affects both gray matter and white matter. De- 

 pending upon the exact level at which the section is taken, the cord may be nearly 

 round or flattened dorso-ventrally. The posterior horns remain slender while 

 the anterior are much broader than the posterior horns. The reticular process is 

 more prominent than in any of the previous sections. As in the lumbar cord, 

 the cell groups of the anterior horn are numerous and well defined. A more or less 



