888 THE FEATURES OF DIFFERENT REGIONS. [BOOK in. 



575. The provisional conclusions at which we have arrived 

 are further, to a certain extent at least, confirmed and extended by 

 a study of the behaviour at the several regions of the cord of the 

 special tracts of white matter described in 567. 



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 shewn 

 in Fig. 104, C 2 C 5 C 8 . From thence downward it diminishes in size, 

 the diminution being especially rapid in the lumbar swelling, 

 Fig. 104, Lj, 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 downwards, 

 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 mechanisms 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 ; aiid 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 

 appropriate 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 particular kind of efferent impulses 

 which we shall speak of as voluntary or volitional 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 

 connected with several of the cranial nerves, much in the same 

 way that it gives off fibres to the spinal nerves. 



We may therefore picture to ourselves this pyramidal tract as 

 starting in the form of a broad sheaf of fibres from a certain 

 district on the surface of one of the cerebral hemispheres. 

 Putting aside for the present any possible increase of the number 

 of fibres by division of fibres (though we have reason to think that 



