536 THE POSTERIOR PITUITARY AND THE NERVOUS SYSTEM. 



adds, "that the supporting frame-work of the sheath consists 

 of a chain of funnels surrounding the axis-cylinder, each fun- 

 nel being formed by a spiral thread." Tizzoni 35 "believes 

 that there is but one net-work closely investing the axis- 

 cylinder, and that it is in connection with the slits of Lauter- 

 mann." McCarthy is stated to have shown that, "after a 

 nerve has been hardened with picric acid and ammonium 

 chromate, the medullary sheath contains minute, rod-like 

 structures, which pass radially between the axis-cylinder and 

 the primitive sheath so as to give the cross-section of a fiber 

 the appearance of a wheel. The rods stain with carmine and 

 haematoxylin, which do not stain the myelin. It is not possible 

 to isolate the rods as separate elements, for they are not dis- 

 tinct from one another, but united." Finally he refers to the 

 fact that Lautermann, von Stilling, Roudanowski, and Mc- 

 Carthy all believe that there is "a system of hollow canals in the 

 sheath of the axis-cylinder," and himself reaches the conclu- 

 sion that the cones they form are protoplasmic, and not com- 

 posed of neuro-keratin, as is usually held. He divides "each 

 cone into six segments placed at regular distances apart and 

 converging from the primitive sheath to the axis-cylinder." 

 This is well shown in the annexed illustration, reproduced from 

 his paper. If we now consider the segments as canaliculi 

 leading from the axis-cylinder, we can readily see how the 

 blood-plasma can penetrate the myelin and its oxidizing sub- 

 stance, and these bodies carry on, when brought into contact, 

 a reaction similar to that which occurs in muscle-fiber. Indeed, 

 if the various features enumerated are collectively considered, 

 it will become apparent that the myelin, or white substance of 

 Schwann, when in contact with the oxidizing substance of the 

 Hood-plasma undergoes a reaction in which chemical energy is 

 liberated. 



When we consider that the axis-cylinder is, as stated, the 

 continuation of a neuron's axon, it is not difficult to account 

 for the various phenomena, known under the general term of 

 "nerve-degeneration," i.e., the disorganization of myelin, the 

 dissolution of the myelin, etc., at the distal end of a nerve, 



'Tizzoni: Archivio per le Sci. med., vol. iii, fasc. 1, 1878. 



