548 THE SPINAL CORD. 



pears to receive a permanent addition at the entrance of each nerve. We 

 may infer that each nerve has a representative of itself starting from the 

 level of its entrance and running up to some part of the brain. Whether 

 the fibres thus representative of the nerve are continuations of the very 

 fibres of the nerve itself, or are new fibres starting from some relay of 

 gray matter, with which the fibres of the nerve are also connected, is another 

 question. 



484. The gray matter in contrast to the white matter shows great varia- 

 tions in area along the length of the cord (Fig. 125). From the entrance 

 of the coccygeal nerve upward the area increases very rapidly,^ reaching a 

 maximum at about the level of the fifth lumbar nerve. It then rapidly de- 

 creases to about the level of the eleventh thoracic nerve, maintains about the 

 same dimensions all through the thoracic region, and begins to increase again 

 at about the level of the second thoracic nerve. Its second maximum is 

 reached at about the level of the fifth or sixth cervical nerve, after which 

 the area again becomes smaller, remaining, however, at the upper cervical 

 region much larger than in the thoracic region. 



FIG. 126. 

 10 



I V IV III II I XII XI X IX VIII VII VI V IV III II I VIII VII VI V IV III II I 



Diagram showing the Relative Sectional Areas of the Spinal Nerves as they Join the Spinal 



Cord. 



The meaning of these variations becomes clear when we turn to Fig. 126, 

 which shows in a similar diagrammatic manner the sectional areas of the 

 several spinal nerves. It will be observed that the increase and decrease of 

 the sectional area of the gray matter follow very closely the increase and 

 decrease of the quantity 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 first and second sacral, fourth and fifth lumbar 

 nerves are very large, and opposite to these the sectional area of the gray 

 matter of the cord is very large also ; the enlargement of gray 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 gray matter which is the essen- 

 tial 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 gray 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 gray 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 gray 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 pres- 

 ent 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 gray 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 



