252 THE ADRENAL SYSTEM AND VASOMOTOR FUNCTIONS. 



But why do the dilator fibers not degenerate likewise? 

 Inasmuch as, quoting Professor Foster's words, "the presence 

 of any vasoconstrictor fibers at all has not at present been 

 satisfactorily established/' while "the only evidence of their 

 existence" is that, "when the nerve of a muscle is divided, the 

 blood-vessels of the muscle widen," we must admit, in the face 

 of the foregoing statements, that all the evidence now tends 

 the other way: i.e., to suggest that the sciatic and the brachial- 

 plexus nerves are not only motor, but "motor-constrictor" 

 nerves, and that dilators are autonomous structures if they 

 exist at all. 



Referring to the effects of severance of a nerve from the 

 central nervous system, Foster says: "When a nerve such, 

 for instance, as the sciatic is divided in situ, in the living 

 body, there is, first of all, observed a slight increase of irrita- 

 bility, noticeable especially near the cut end; but after awhile 

 the irritability diminishes and gradually disappears. Both the 

 slight initial increase and the subsequent decrease begin at the 

 cut end and advance centrifugally toward the peripheral ter- 

 minations. This centrifugal feature of the loss of irritability 

 is often spoken of as the Hitter- Valli law. In the mammal it 

 may be two or three days; in a frog, as many, or even more 

 weeks, before irritability has disappeared from the nerve-trunk. 

 It is maintained in the small (and especially in the intra- 

 muscular) branches for still longer periods." This obviously 

 suggests that the size of a nerve, all else being equal, is a gov- 

 erning factor in the degenerative process due to nerve-section, 

 precisely as indicated when the relative effects of the electric 

 current were referred to. 



Still, such a governing principle would necessitate that a 

 large nerve be structurally similar to a small one, qualitatively 

 and quantitatively, to warrant our accepting it as the basis of 

 a final conclusion. Such is not the case, however, as is well 

 known. While the various structures that enter into their 

 formation are specific to nervous organs, they are not evenly 

 distributed. This is illustrated in the case of vasomotor nerves. 

 Though both constrictors and dilators are medullated, the 

 former lose their medulla early in their course, while the vaso- 

 dilators preserve theirs until the blood-vessels to which they 



