NERVOUS TISSUES. 



181 



of the muscle, the rate can be demonstrated at which the 

 nervous influence or impression travels. The rate is estimated 

 at more than 100 feet per second in the human subject; but 

 it is known to vary at different times in the same nerve. 



136. Nervous Tissues. Nerve-fibres present considerable 

 variety of microscopic appearance. But those which are 

 largest, and found most abundantly in nerve trunks taking 

 origin from the brain and spinal cord, are the medullated 

 fibres. These may have a diameter as great as YTTTTF ^ an 

 inch. They present a limiting membrane, with nuclei 



3 



Fig. 95. NERVE-FIBRES. , Medullated fibre in the fresh state ; 



b, similar fibre showing the nuclei of the limiting membrane ; 



c, axis -cylinder projecting beyond the torn limiting membrane of 

 a fibre, from which the medullary sheath has been partially 

 removed ; d, substance of medullary sheath escaping in irregular 

 drops ; e, fibre treated with acetic acid ; /, fine medullated fibre 

 from brain, the medullary sheath running into drops, being un- 

 supported by any limiting membrane ; g, grey fibres. 



scattered on it, like the sarcolemma of muscle, and inside of 

 this, in the fresh state, a clear and seemingly homogeneous 

 substance, which oozes out in a semifluid fashion from places 

 where the limiting membrane is ruptured. But after a short 

 time, or on addition of reagents, such as a drop of acetic acid, 

 a coagulation takes place, and there are then exhibited 

 a thin central thread, the axis-cylinder, in the middle, and 

 around this a substance which is called from its opacity when 



