554 THE SPINAL CORD. 



anterior. This shifting goes on through the cervical region up to about the 

 level of the second cervical nerve, where it is arrested by the beginning of 

 the changes through which the spinal cord is transformed into the far more 

 complicated bulb. 



This lengthening of the posterior fissure indicates an increase in the 

 dorso-ventral diameter of the posterior columns, and this, not being accom- 

 panied by a compensating diminution of the side-to-side diameter, shows in 

 turn that the posterior columns undergo an increase in passing upward. 

 From this we may add to the provisional conclusion just arrived at with 

 regard to the lateral columns, the further conclusion that some part of the 

 posterior columns also is concerned in transmitting impulses, in a more or 

 less direct manner, between the various regions of the cord below and the 

 brain above. The anterior columns do not increase in the same marked 

 manner, though over and above the increase due to the lumbar and cervical 

 swellings a continued increase may be observed, especially in the upper cer- 

 vical region ; it is in this upper region that the direct pyramidal tract is 

 best developed. 



488. The provisional conclusions at which we have arrived are further, 

 to a certain extent at least, confirmed and extended by a study of the 

 behavior at the several regions of the cord of the special tracts of white 

 matter described in 479. 



The pyramidal tract, that is to say, the crossed pyramidal tract entering 

 the spinal cord above from the pyramid, is very large in the cervical region, 

 having the form and situation shown in Fig. 1 27, C 2 C 3 C 8 . From thence 

 downward it diminishes in size, the diminution being especially rapid in the 

 lumbar swelling (Fig. 127, A), where the tract, being no longer covered in 

 by the cerebellar tract, comes to the surface of the cord ; but it may be 

 traced by the degeneration method down as far as the coccygeal region, and 

 indeed appears to be coexistent with the entrance of spinal nerves into the 

 cord. Diminution of the tract means a lessening of the number of fibres; 

 and since we cannot suppose that any of the fibres come suddenly to an end 

 in the tract itself, we are led to infer that along the cord, from above down- 

 ward, fibres are successively leaving the tract and passing to some other part 

 of the cord. We seem further justified in concluding that the fibres which 

 thus successively leave the tract go to join the series of local nervous mech- 

 anisms with which the spinal nerves communicate, as we have seen reason 

 to believe, upon their entrance into the cord. Indeed, as we shall see later 

 on, we have reason to think that the nervous mechanisms which the fibres in 

 question join are those belonging to the motor fibres of the anterior roots. 

 This pyramidal tract does not begin in the pyramid, but may be traced 

 through the lower parts of the brain right up to special areas in the cortex 

 or surface of the cerebral hemispheres ; and very strong reasons may be 

 brought forward in support of the view that the fibres of this tract are 

 fibres which carry impulses from the cortex to successive portions of the 

 spinal cord and there give rise to efferent impulses which pass to appro- 

 priate skeletal muscles. The tract, therefore, is not only a descending 

 tract by virtue of the mode of degeneration, but may be spoken of in a 

 broad sense as a tract of efferent impulses descending from the cerebral 

 cortex ; and, indeed, it is maintained that it is the channel of the partic- 

 ular kind of efferent impulses which we shall speak of as voluntary or voli- 

 tional impulses. We may add that as the tract passes along a path, which 

 we shall subsequently describe, from the cerebral cortex through the lower 

 parts of the brain to the pyramid, it gives off fibres to mechanisms con- 

 nected with several of the cranial nerves, much in the same way that it 

 gives off fibres to the spinal nerves. 



