692 THE FEATURES OF DIFFERENT REGIONS. [Book hi. 



sectional areas of the several spinal nerves. It will be observed 

 that the increase and decrease of the sectional area of the grey 

 matter follow very closely the increase and decrease of the quan- 

 tity of nerve, that is to say, neglecting differences in the diam- 

 eter of the fibres, in the number of nerve-fibres passing into the 

 cord. The sectional areas of the 1st and 2nd sacral, 4th and 5th 

 lumbar nerves are very large, and opposite to these the sectional 

 area of the grey matter of the cord is very large also ; the en- 

 largement of grey matter which is the essential cause of the 

 lumbar swelling is correlated to the large number of fibres which 

 enter and leave the cord at this region to supply chiefly the lower 

 limbs. Similarly the enlargement of grey matter which is the 

 essential cause of the cervical swelling is correlated to the large 

 number of fibres which enter and leave this region of the cord 

 to supply chiefly the upper limbs. In the thoracic region, where 

 the number of fibres entering and leaving the cord is relatively 

 less, the sectional area of the grey matter is also less. Since 

 the attachments of the several spinal nerves are not exactly 

 equidistant from each other along the length of the cord, the 

 sectional area is not an exact measure of bulk ; the total bulk 

 of grey matter for instance belonging to two nerves which enter 

 the cord close together is less than that of two nerves giving 

 rise to the same sectional area of grey matter as the former two 

 but entering the cord far apart from each other. Still the error 

 which may be introduced by taking sectional area to mean bulk 

 is, for the present purposes at all events, so small that we may 

 permit ourselves to say that in the successive regions of the 

 spinal cord the bulk of grey matter in any segment is greater 

 or less according to the size of the nerve (or pair of nerves, right 

 and left) belonging to that segment. 



From this anatomical fact we appear justified in drawing 

 the conclusion that at all events a great deal of the grey mat- 

 ter of the spinal cord may be considered as furnishing a ner- 

 vous 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 grey 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 of core of grey mat- 

 ter, 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 grey matter more immedi- 

 ately connected with the fibres of the nerves. This question 

 the method which we are now using cannot settle. 



§ 455. Owing to these different rates of increase of the 

 grey 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 man- 



