800 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



It is thus seen that the white fibres of the spinal cord are not in 

 direct communication with the nerve-roots, but only connect different 

 segments of the spinal cord. In each segment of the spinal cord are to be 

 found nerve-centres for the afferent and efferent nerves of definite regions 

 of the bod j', and commissural fibres connecting that segment with other 

 segments of the cord and with the brain (Fig. 342). 



Leaving these anatomical considerations of the moment, we have 

 now to examine the data as to the paths of conduction of motor and 

 sensory impulses through the spinal cord attained through experiments 

 performed upon the spinal cord. The method of such experiments con- 

 sists in attempting to make isolated sections of different parts of the 

 cord and determine the interference of function which results from such 

 mutilations. The difficulty of such experiments is twofold : In the first 



Fig. 342.— Diagram illustrating the Various Channels through 

 "which a Motor Cell of the Cord may be Called into Action. 

 (Ranney.) 

 A.H, anterior horn ; C.P.C, crossed pyramidal column : P, posterior horn; B, column of Burdach; 



G, column of Goll ; 1, fibre for painful sensations ; 2, fibre for tactile sensations ; 3, motor cell ; 4, motor 



fibres ; 5, fibre from opposite cerebral hemisphere going to cell, 3 ; 6, ganglion on posterior nerve-root ; 



7, fibre from cerebral hemisphere of same side going to motor cell, 3. 



place, it is difficult to separate the temporary result, due to the shock of 

 the operation, and the permanent results ; and, in the case of sensation in 

 the lower animals, to distinguish between reflex action and purely volun- 

 tary movements. In the second place, it is almost impossible to make a 

 section of any isolated portion of the spinal cord without more or less 

 damaging the adjacent portions. Thus, for example, it is almost impos- 

 sible to cut transversely the posterior white columns of the cord without 

 at the same time more or less injuring the gray matter. This difficulty 

 applies to isolated sections attempted on all parts of the spinal cord. 

 Nevertheless, a number of valuable results have been obtained by this 

 method. 



In the first place, it has been found that section of the cord causes 

 immediate loss of both sensation and motion in all parts supplied by 



