EXPERIMENTS ON VERTEBRATES 135 



as has been said above, suggest externally its segmental 

 character. Each has a ventral motor and a dorsal 

 sensory root, and we desire to call attention to the 

 fact that the dorsal root passes through a ganglion 

 (the spinal ganglion). If the ventral root be severed, 

 paralysis of the muscles of the corresponding segment 

 occurs. If the dorsal root be severed, the correspond- 

 ing segment becomes insensible, or, more properly 

 speaking, the transmission of impulses which proceed 

 from the periphery to the muscles of this segment and 

 to the remaining segments becomes impossible. The 

 operation itself, however, has still another influence 

 on the tension of the muscles of the same segment 

 (perhaps also of other segments). The amount of the 

 muscle-tension under normal conditions varies (prob- 

 ably with the chemical conditions of the muscles). If 

 a muscle be stretched with a certain weight it attains a 

 certain length. But if the posterior nerve-roots be 

 severed while the muscle is still nervously connected 

 with its segment the muscle lengthens (E. v. Cyon). 

 The operation causes a shock, in other words, prob- 

 ably a chemical change in the muscle. The nature of 

 this change is as yet unknown. This influence of the 

 posterior roots on the muscles shows itself also in the 

 movements of an animal in which the posterior roots 

 of the hind-legs have been severed : the movements of 

 the legs are disturbed. It is known that the nerves 

 of the brain are also of segmental origin, only in this 

 case the inequalities of growth obliterate externally 

 the segmental relations. From the fact that the chiefly 



