REFLEX PROCESSES 575 



Moreover the central end of one efferent nerve can be united with the 

 peripheral end of another. Two afferent nerves can likewise be crossed in 

 the same fashion; but it has not yet been decided how far afferent nerves 

 mediating different kinds of sensations can be joined together. It appears 

 that no union of afferent with efferent nerves is possible. 



Several authors reject the view that an actual regeneration takes place in 

 a nerve which has been completely separated from the central nervous system, 

 and Bethe has reported a number of interesting experiments which tend to 

 discredit that view. Other authors have observed in the peripheral stump 

 of a cut nerve the appearance of spindle-shaped cells lying in the longitudinal 

 direction of the nerve. Should it be shown beyond a doubt that axis cylinders 

 develop within these structures we could no longer regard axis cylinders as 

 processes of the nerve cells,, and the nutritive influence over the nerve fibers 

 ascribed to the nerve cell would sink to a matter of small importance. In 

 short, our whole conception of the structure and functions of the nervous 

 system would of necessity be very profoundly modified. Langley, however, 

 on the basis of some experiments of his own, believes that the nerve fibers 

 which apparently arise de novo actually grow into the peripheral stump from 

 nerves of the surrounding tissue and eventually trace back to nerve cells of 

 the spinal cord. It is to be observed also that Bethe himself finds great varia- 

 tion in the number of autochthonous * fibers, since never in his experiments 

 did all the fibers regenerate in this fashion. Besides, this form of regeneration 

 appeared at its strongest only in ) r oung animals; in grown animals the fibers 

 stopped, as Bethe puts it, " halfway," not being powerful enough of them- 

 selves to complete the regeneration without the help of the spinal cord. For 

 the present the matter cannot be regarded as settled in Bethe's favor. 



Langley has contributed some very important results on the regeneration of 

 sympathetic nerves, but for several reasons it seems best to discuss these in con- 

 nection with the subject of physiology of special nerves (Chapter XXV). 



5. REFLEX PROCESSES 



A. SEGMENTATION IN THE CENTRAL NERVOUS SYSTEM 



We learn from the anatomy of the lower vertebrates (lamprey, salaman- 

 der) that the nerve fibers from the spinal roots, in certain sections at least, 

 run but a short distance up or down the spinal cord i. e., that there is here 

 an evident segmentation of the spinal cord. 



Likewise in the higher vertebrates the spinal cord can be regarded as to 

 a certain extent made up of serially homologous parts, connected together in a 

 very complex manner. Each such segment consists of a pair of nerve roots 

 together with that portion of the spinal cord belonging to them and consti- 

 tutes in itself the simplest kind of a central organ. 



Because of the many short- and long-fibered pathways uniting the separate 

 segments of the spinal cord with each other and with the different portions 

 of the brain, impulses arising in the different segments have so many ways 



1 That is. originating in the place where they are found. 



