ranvier's constrictions in the spinal cord. 93 



tage in the frog and the guinea-pig ; but the crosses are larger 

 and clearer in the spinal cord of the ox. 



Osmium Preparations — teased. — Difficulties surround 

 the demonstration of constrictions in teased preparations of 

 central fibres treated with osraic acid alone. Only a small 

 number of the fibres are properly '' fixed. ^^ The absence of 

 Schwann's sheath makes isolation difficult, and from the same 

 cause by far the greater number of the constrictions are torn 

 by the needles. We have no reason to suppose that the rule 

 according to which the interval between any two constrictions 

 is approximately directly proportional to the thickness of the 

 fibre is any less true of central than of peripheral fibres. It is 

 the larger central fibres which are most easily isolated^ and 

 their constrictions, if this rule holds good, must be widely 

 separated. In the animals used by me it seemed impossible to 

 isolate pieces long enough to show two constrictions. Only 

 when the locus miuorae resistentiee has escaped the teasing 

 needles and the fibre is torn through the myelin can constric- 

 tions be found. If the teasing is not too thoroughly done, 

 small groups of fibres, lying for the most part parallel, will be 

 secured. It is near the edges of these groups that the con- 

 strictions are most likely to be found, because the outermost 

 fibres protect the rest. In both rabbit and ox I have seen 

 entirely isolated fibres with good constrictions. 



Osmic Acid Preparations; Sections. — Longitudinal 

 sections of central fibres are best made from the spinal cord of 

 the ox. The part used by me was taken from the neighbour- 

 hood of the median fissure. For the detection of the con- 

 strictions sections of the average thickness of the fibres are 

 most suitable. The constrictions are not so easily seen when 

 the nerves are cut in the plane of the axis-cylinder. Only the 

 outer fibres of a section should be used, because they are 

 usually found lying with more space between each fibre than 

 is the case elsewhere, and are better " fixed." 



Many of the interruptions in the continuity of the nerves 

 are of considerable width. They are so broad that one at once 

 thinks of artificial separations or of places where fibres have 



