Irving Hardesty 343 



nears completion only the nucleus appears adhering to the periphery of 

 the sheath. The fact that even such nuclei are rare in the adult suggests 

 that they also may disappear. 



5. In the relative abundance of their protoplasm at the different 

 periods, these cells resemble the nerve corpuscles described for the per- 

 ipheral nerve fibers and which are interpreted as having to do with the 

 structures of the sheath. 



Ox THE Framework of the Medullary Sheaths of the Spinal 



Cord. 



In none of my preparations of the spinal cord of the hog is there evi- 

 dent any arrangement in the medullary sheaths producing the appearance 

 of the Schmidt-Lantermann clefts and segments described in the sheaths 

 of the peripheral libers. Especially is there no evidence of the heavy, 

 separate interfitting cones described by Wynn. Hatai, who used a 

 method much superior for the purpose to that used by Wynn, describes 

 the structure of the peripheral medullary sheath as consisting of a net- 

 work which contains the myelin. This " neurokeratin " network con- 

 sists, he says, first, of two thin layers, one beneath the sheath of Schwann, 

 and the other around the axone, the two being continuous at the nodes 

 of Eanvier ; and second, of a chain of cone-like formations, the bases of 

 the cones being attached to the peripheral layer and the apices to the 

 axone layer. Both the cones and the layers are highly reticular, exhib- 

 iting meshes of various sizes and shapes. He thinks the Schmidt-Lan- 

 termann clefts are produced artificially. 



Hatai found formalin the best fixing agent in his study of this frame- 

 work. Formalin is the fixing agent required in the Benda method and the 

 alizarin used in the staining procedure of this method apparently brings 

 out the framework of the medullary sheath in greater delicacy of detail 

 than the stain used by Hatai. The toluidin blue of the Benda method 

 stains the axone a dense blue as it does the developed neuroglia fibers, 

 while the white fibrous connective tissue and the framework of the medul- 

 lary sheaths is stained a light brownish-purple even to the finest fibrillai. 

 After formalin, as after many other fixing agents, the axone shrinks to 

 considerable density and decrease in its normal diameter, but on the other 

 hand, formalin seems to produce a slight swelling of the medullary sheath. 

 In this process the framework of the sheath remains attached to the 

 axone and is thus drawn or distended into a more open condition which 

 renders the study of its detailed structure less difficult. 



On comparing sections of peripheral nerve fibers with sections of the 



