The Journal of 

 Comparative Neurology and Psychology 



Volume XV 1905 Number 1 



ON THE AREAS OF THE AXIS CYLINDER AND 

 MEDULLARY SHEATH AS SEEN IN CROSS 

 SECTIONS OF THE SPINAL NERVES OF VER- 

 TEBRATES. 



By Henry H. Donaldson and G. W. Hoke. 



{From the .Veurologkal Laboratory of the University of Chicago.) 

 With one figure. 



Introduction. — The results presented in this paper are, in 

 each case, based on averages of the measurements of twenty 

 or more spinal nerve fibers. The nerves were taken from vari- 

 ous animals representing the five great classes of vertebrates. 

 The measurements show that the areas of the medullary sheath 

 and enclosed axis are nearly equal, and by consequence that 

 the volume of the substance forming the axis cylinders is equal 

 to that forming the medullary sheaths. The relation con- 

 stitutes a point of similarity remarkable for its wide extension 

 through the vertebrate series.' It enables us, moreover, to 

 estimate in any nerve the volume of the substance specialized for 

 the conduction of the nerve impulse. Since this quantitative 

 relation between axis cylinder and sheath is so close, it strongly 

 suggests that in some way the axis controls the formation of 

 the surrounding medullary substance. 



In the spinal nerves of some animals this relation, as ex- 

 pressed by the equal areas of the axis and sheath when the fibers 

 are seen in the cross section, was pointed out several years ago 



^ The Acrania and Cyclostorai do not develop medullary sheaths on their 

 nerve fibers, and are therefore not included. 



