884 THE FEATURES OF DIFFERENT REGIONS. [BOOK in. 



grey matter does not affect all parts of the grey matter alike, so that 

 the outline of the grey matter changes very markedly in passing 

 from below upwards. In the coccygeal region each lateral half is 

 a somewhat irregular oval, and in the sacral region, Fig. 104, Sac, 

 the differentiation into anterior and posterior horns is still very 

 indistinct. In the lumbar region the two horns are sharply marked 

 out, though both the posterior and anterior horns are broad and 

 more or less quadrate. In the thoracic region the decrease of 

 grey matter has affected both horns, so that both are pointed and 

 slender, while the junction between them has not undergone so 

 much diminution, so that what has been called the lateral horn 

 is relatively conspicuous. In the cervical region the returning 

 increase bears much more on the anterior horn which again becomes 

 large and broad, than on the posterior horn which still remains 

 slender and pointed. Taking the form of the grey matter in the 

 thoracic region as the more typical form of the grey matter we 

 may say that while the increase on the lumbar swelling bears 

 equally on the anterior and posterior horns, that in the cervical 

 region bears chiefly on the anterior horns. 



Now we have no reason to suppose that either afferent 

 impulses reach the lumbar spinal cord in greater numbers from 

 the lower limbs, or along any of the nerves joining this part of 

 the cord, or that those which do reach it are of a more complex 

 nature than is the case with the afferent impulses reaching the 

 cervical cord along the nerves of the upper limbs. The increase 

 of grey matter in the posterior horns is therefore not correlated 

 to any increase in the number or complexity of the afferent 

 impulses reaching the cord ; and we may, provisionally conclude 

 that at least a large part of the grey matter in the posterior 

 horn is not specially concerned in any elaboration or transformation 

 of afferent impulses immediately upon their arrival at the cord. 

 Indeed we have seen that while there is ample evidence to connect 

 the nerve cells, and therefore presumably the grey matter in 

 general of the anterior horn with the efferent motor fibres of the 

 anterior root, there is no corresponding evidence as to any large 

 immediate connection of the afferent fibres of the posterior root 

 with the nerve cells or indeed any other part of the grey matter of 

 the posterior horn. We may add that, as we shall point out later 

 on, so essential is the concurrence of appropriate afferent impulses 

 to the due carrying-out of complex coordinate motor or efferent 

 impulses, that we can scarcely expect to find any increase in the 

 nervous mechanisms devoted to the purely motor function of 

 carrying out motor impulses without a corresponding increase in 

 the nervous mechanisms belonging to the afferent impulses, by 

 means of which those motor impulses are guided and coordinated. 

 Hence, were the latter nervous mechanisms restricted to the 

 posterior horns we should expect to find a greater parallelism than 

 does actually exist between them and the anterior horns. 



