138 ELEMENTARY BIOLOGY. [CHAP. 



?. Schmidt's nodes. Examine the medullary 

 sheath of the least distorted fibres with great 

 care ; the above will be found as interruptions 

 of the same at short and fairly regular in- 

 tervals, the sheath appearing to consist of a 

 series of imbricated segments. 



17. Compare a fibre with irregular contour. Note 

 that the irregularity is due to shrinking of 

 the medullary sheath under coagulation. The 

 coagulate masses may often be seen to cor- 

 respond with the above-named segments. 



iii. Tease up a piece of the trunk of a spinal nerve 

 in a drop of 1 per cent, osmic acid solu. After 

 3 — 4 hours immersion in the same, wash well 

 and mount in weak glycerine. Examine under 

 a high power. 



a. The medullary sheath ; rendered blackish and 

 highly conspicuous by the above method. 

 Examine it with care, noting especially the 

 nodes of Ranvier and Schmidt. 



ft. The axis fibre ; greyish and well defined, 

 seen on focussing through o. 



y. The nerve corpuscles ; apparent as thickenings 

 of the sheath which bulge inwards, usually 

 of a reddish brown colour under treatment 

 with the above acid. They correspond in 

 number with the nodes of Ranvier, and each 

 consists of an oval refractive nucleus, em- 

 bedded in a small amount of granular proto- 

 plasm. 



