CHAP, i.] THE SPINAL CORD. 881 



of nerve, that is to say, neglecting differences in the diameter 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 enlargement 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 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 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 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 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 grey 

 matter more immediately connected with the fibres of the nerves. 

 This question the method which we are now using cannot settle. 



572. 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 manner. This is 

 strikingly shewn by comparing Figs. 96, 98 and 99. At the level 

 of the third lumbar nerve (Fig. 99) the grey 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. 



562 



