THE STRUCTURE OF THE SPINAL CORD. 549 



that at all events a great deal of the gray matter of the spinal cord may be 

 considered as furnishing a nervous mechanism, with which the efferent fibres 

 of each spinal nerve just before they leave the cord, and the afferent fibres 

 soon after they join the cord, are more immediately connected. It may be 

 that the whole of the gray matter is thus directly connected with and thus 

 rises and falls with the fibres of the nerves ; or it may be that there is a sort 

 ofv cord of gray matter, which maintains a uniform bulk along the whole 

 length of the cord and serves as a basis which is here more and there less 

 swollen by the addition of the gray matter more immediately connected with 

 the fibres of the nerves. This question the method which we are now using 

 cannot settle. 



485. Owing to these different rates of increase of the gray and white 

 matter respectively along the length of the cord, we find that in sections of 

 the cord taken at different levels the appearances presented vary in a very 

 distinct manner. This is strikingly shown by comparing Figs. 119, 121 and 

 122. At the level of the third lumbar nerve (Fig. 122) the gray matter is 

 very large, reaching, as we have seen, its maximal sectional area at about 

 this point, so that although the area of white matter is not very great the 

 whole area of the cord is considerable. 



At the level of the sixth thoracic nerve (Fig. 119), in spite of the white 

 matter having very decidedly increased, the gray matter has shrunk to such 

 very small dimensions that the total sectional area of the cord has markedly 

 diminished. 



At the level of the sixth cervical (Fig. 121) the gray matter has again 

 increased, reaching here, as we have seen, its second maximum ; the white 

 matter has also further increased, and that indeed very considerably, so that 

 the total area of the cord is much greater than in any of the lower regions. 



Further details of the varying size of the white matter and of the gray 

 matter at different levels are also shown in the series given in Fig. 127. In 

 these, combined with the three figures just referred to, it will be observed 

 that the serial increase and decrease of the gray matter does not affect all 

 parts of the gray matter alike, so that the outline of the gray matter changes 

 very markedly in passing from below upward. In the coccygeal region each 

 lateral half is a somewhat irregular oval, and in the sacral region (Fig. 127, 

 Sac.) the differentiation into anterior and posterior horns is still very indis- 

 tinct. 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. 

 Iii the thoracic region the decrease of gray 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 gray matter in the thoracic region as the more typical form 

 of the gray matter we may say that while the increase in the lumbar swell- 

 ing 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 gray matter in the posterior horns is therefore not correlated to any in- 

 crease 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 



