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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



For the most complex voluntary reactions the entire central system is 

 necessary, and especially the cortex of the cerebral hemispheres, while it has 



already been shown that the impulses 

 which cause reflex actions can make 

 their circuit in a very limited portion 

 of the spinal cord. In the case of vol- 

 untary reactions the impulses take a 

 longer pathway and involve a larger 

 number of nerve-elements, since from 

 the point at which they enter the sys- 

 tem they must pass to the cephalic end. 

 At the same time, in a voluntary reac- 

 tion a greater number of impulses com- 

 bine to modify the discharge from the 

 efferent cells. 



Tracts in the Central System. 

 How this result is accomplished has 

 been studied both in mammals and in 

 man. Histology shows us the fibres of 

 the dorsal root entering the cord and 

 sending one branch cephalad and the 

 other caudad, both branches giving off 

 collaterals (Fig. 174). In man and the 

 higher mammals the dorsal root-fibres 

 enter the cord in three groups a me- 

 dian group, an intermediate group of 

 large fibres, and a lateral group of 

 very fine fibres, the bundle of Lissauer. 

 When the dorsal root is sectioned be- 

 tween the ganglion and the cord, all 

 these fibres degenerate. 

 The degeneration extends in the dorsal columns down the cord two or 

 three centimeters from the level of the section, and also up the cord as far as 

 the nuclei of the dorsal columns, located at the commencement of the bulb. 

 If the section is made near the caudal end, the degeneration may in conse- 

 quence run through the entire length of the cord. Moreover, it occurs only 

 on the side of the cord to which the sectioned nerves belong. Take, for 

 example, the area of degeneration caused by the section in a dog of the dorsal 

 roots on the left side between the sixth lumbar and second sacral nerves. 

 The degeneration in the lower lumbar region is represented in Figure 175, A, 

 in the upper lumbar region in _B, and in the thoracic in C. On passing 

 cephalad the area of degeneration becomes smaller. This is interpreted to 

 mean that all along, between the caudal and cephalic limits, fibres are given 

 off from the main bundle to the intermediate segments of the cord. Here is 

 evidence of an arrangement that is always to be kept in view. Though a 



FIG. 174. Schema showing pathway of the sen- 

 sory impulses. On the left side S, S' represent 

 afferent spinal nerve-fibres ; C, an afferent cranial 

 nerve-fibre. This fibre in each case terminates 

 near a central cell, the neuron of which crosses 

 the middle line and ends in the opposite hemi- 

 sphere (van Gehuchten). 



