CHAP, i.] THE SPINAL CORD. 889 



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 injuries to particular areas of the cerebral cortex, the main 

 mass of degenerated fibres, after crossing over from one side of 

 the cerebrospinal 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 degene- 

 ration 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. 104, dP), except that it does 

 not cross at the decussation of the pyramids, is otherwise similar 

 to the crossed pyramidal 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 particular 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. 104, D 6 D 8 . Taking an average we may say that, of the 

 whole strand running in the pyramids above the decussation, 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 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 



