266 -1 Manual of Veterinary Physiology. 



When the various tracts in the spinal cord reach the 

 medulla they undergo change in form, position, and distri- 

 bution, in order that they may arrive at the various parts 

 of the brain to which they are proceeding. This will be 

 alluded to again when speaking of the medulla ; but we 

 may here mention that only two unbroken spinal tracts pass 

 through the medulla to the higher centres, all the others 

 being broken up. 



The conducting paths in the cord will be studied pre- 

 sently, when we have learned the functions of the spinal 

 nerves and of the nerve centres. 



Spinal Nerves. — We must now describe very briefly how 

 the nerves entering and leaving the spinal cord are disposed 

 with reference to the cord itself. 



The superior or sensory roots enter the cord just by 

 the upper cornu ; some of the fibres run up, and some 

 down the cord for a short distance, and then pass hori- 

 zontally across it. Other fibres enter the white instead 

 of the gre} r substance, and after passing some distance up 

 the superior columns of the cord enter the grey matter. 

 The connection between the sensory nerves and the cells 

 of the grey matter of the superior cornu, is by no means 

 so definite as the connection of the motor nerves with the 

 grey matter of the inferior cornu. The motor nerve, or 

 inferior root, is continuous with the axis cylinders of the 

 cells of the inferior horn, though some fibres may be 

 found to be in connection with the white matter of the 

 lateral columns ; these later on cross over the cord and enter 

 the grey matter of the inferior cornu of the opposite side. 



The chief difference in the behaviour of the two roots is 

 that the inferior is in direct connection by means of their 

 axis cylinders with the nerve cells of the inferior cornu, 

 whilst the superior root is only in connection with the 

 nerve cells of the superior cornu, by moans of a network of 

 nerve tissue known as Oerlach's. 



If the sensory root before entering the cord he divided 

 above the ganglion, its division causes pain : irritation of 

 the distal extremity causes no sensation, whilst irritation 



