1903.] on the Paths of Volition. 273 



substance, which is known as the brain stem, in which they lie at the 

 surface in a ventral situation, forming the greater part of the mass of 

 nerve fibres, which has been termed the foot of the stem (pes pedun- 

 culi) or crusta ; through the pons Varolii, where they are covered 

 and concealed by the fibres which emerge from the hemispheres of 

 the cerebellum ; and down the ventral part of the medulla oblongata, 

 where they form two bulging projections on either side of the middle 

 line. These projections were termed by the older anatomists the 

 anterior pyramids, and this name has given rise to the designation 

 pyramidal tracts for the fibres which we are tracing and which are 

 contained within these pyramids. Finally, from the pyramids of the 

 medulla oblongata we can trace these same fibres crossing the middle 

 line in large masses ; and passing into the upper part of the spinal 

 cord, where they are found to have deserted the ventral or anterior 

 surface to which in the medulla oblongata they were confined, and to 

 lie deeply in the nervous substance, occupying a large triangular area 

 in the opposite lateral column of the cord The general structure 

 of the parts which have just been enumerated is shown in the 

 accompanying photographs (8 to 17), and the manner in which the 

 Wallerian degeneration can be followed by the Marchi method is 

 illustrated in the slides which I now show you, which are made from 

 preparations by Dr. Sutherland Simpson, and illustrate the degenera- 

 tions which occur in the pyramidal tract of the cat as a result of injury 

 or removal of the motor cortex of the brain. Not all the fibres in the 

 pyramidal tract, however, go into the opposite lateral column of the 

 cord. A few pass into the lateral column of the same side, and a 

 variable number, in man and the anthropoid apes (but Hot in any 

 other animals), remain for a while in the neighbourhood of the ventral 

 or anterior fissure, and only gradually disappear from it as we trace 

 them down from the cervical region. Ever since this great pyramidal 

 tract of fibres has been recognised, it has been held that along it must 

 pass those volitional impulses which originate in the grey matter of 

 the brain, and which produce the movements of the muscles which 

 are innervated from the spinal cord. And indeed this apj)ears to be 

 established by incontestible evidence, for it is certain that a lesion in 

 any part of the pyramidal tract produces a corresponding jDaralysis 

 in the lower regions of the body. If the lesion be above the place 

 where the pyramidal fibres cross to the opjDosite side (at the junction 

 of the medulla oblongata and spinal cord), the paralysis will be a 

 paralysis of the opposite half of the body. If the lesion affect the 

 fibres of the pyramidal tract below the decussation, the paralysis will 

 be upon the same side. 



We have seen that these pyramidal tract fibres originate in cells 

 of the cortex of the brain, and that they terminate v/ithin the grey 

 matter of the spinal cord, and within corresponding parts of the 

 medulla oblongata, pons Varolii and middle brain. We have also seen 

 that the impressions which they carry emerge from the motor nerve 

 cells in the grey matter of the cord and pass out along the motor 



