THE STRUCTURE OF THE SPINAL CORD. 555 



We may, therefore, picture to ourselves this pyramidal tract as starting 

 in the form^f a broad sheaf of fibres from a certain district on the surface 

 of one of the cerebral hemispheres. Putting aside for the present any pos- 

 sible increase of the number of fibres by division of fibres (though we have 

 reason to think that this does, to a certain extent, occur), we may regard 

 the tract as being at its maximum at its beginning in the cortex. As it 

 descends to the decussation of the pyramids in the bulb it loses a certain 

 number of fibres, which pass off to the cranial nerves. Having crossed and 

 entered into the lateral column of the cord it continues to give off fibres to 

 the spinal nerves, probably to the anterior root of each in succession, and 

 so goes on its way down the cord continually diminishing until the last 

 remaining fibres are given off to the last coccygeal nerve. 



When degeneration is set up along this tract, as may be done by in- 

 juries to particular areas of the cerebral cortex, the main mass of degene- 

 rated fibres, after crossing over from one side of the cerebro-spinal axis to 

 the other in the decussation of the pyramids at the lower end of the bulb, 

 during its further progress down the spinal cord, keeps to the side to which 

 it has crossed right down to the end. Hence, as we have said, it is called 

 the crossed pyramidal tract. The main mass of fibres, the degeneration of 

 which has been started by injury to the left side of the brain, crosses over to 

 the right side of the spinal cord and runs down the lateral column of the 

 right side to the end of the cord. Nevertheless, some fibres appear to cross 

 over again in the spinal cord and then to run along the same side as the side 

 of the brain injured along the left side in the case just mentioned. Such 

 fibres are spoken of as " recrossed fibres." 



The direct pyramidal tract (Fig. 127, dP), except that it does not cross 

 at the decussation of the pyramids, is otherwise similar to the crossed pyra- 

 midal tract, and, indeed, is a part of the same strand to which the crossed 

 tract belongs. When degeneration in this tract is started by injury to par- 

 ticular areas of the cerebral cortex, say on the left half of the brain, the 

 degeneration may be traced through the left anterior pyramid, and so to the 

 left median anterior column of the spinal cord. The direct tract is never so 

 extensive or marked as the crossed tract, does not reach so far down, is much 

 more variable both in length and in sectional area, and, as we have said, is 

 almost confined to man. Diminishing as it descends it may be said to cease 

 in the middle thoracic region (Fig. 127, A A)- Taking an average, we may 

 say that, of the whole strand running in the pyramids above the decussa- 

 tion, about three-fourths of the fibres go to form the crossed and about one- 

 fourth to form the direct tract. We shall see later on that the impulses 

 coming down along the united tract in the brain may, broadly speaking, 

 be said to cross over wholly from one side to the other before they reach 

 the skeletal muscles, so that the impulses passing along the fibres in, say, 

 the left pyramid, reach the muscles of the right limbs and right side of the 

 body, whether the fibres cross over at the decussation to form the crossed 

 or remain on the same side to form the direct pyramidal tract. We are, 

 therefore, led to infer that the fibres in the direct tract, as they pass down 

 the cord, cross over in the cord itself before they make connections with the 

 fibres of the anterior roots. Probably the crossing is effected by means of 

 some of the decussating fibres which form the anterior white commissure. 

 A part only, indeed a small part, of the commissure can serve this purpose ; 

 most of the fibres of the commissure, and in the lower regions of the cord, 

 where the direct tract no longer exists, all the fibres, must have some other 

 functions. Some of the fibres of this great pyramidal tract leave the tract, 

 as we have said, to join some of the cranial nerves before the pyramids of 

 the bulb are reached ; and the impulses passing along these fibres also cross 



