ORGANIZATION 239 



protoplasm. In the higher animals each thread, known 

 as the axis cylinder, 1s surrounded by a delicate, trans- 

 parent covering called the neurilemma, analogous to the 

 sarcolemma of muscle tissue. In the vertebrates, the 

 protoplasmic threads found in many parts of the nervous 

 system have an additional covering made of fatty 

 material, which lies between the axis cylinder and the 

 neurilemma, and is known as the medullary sheath. 

 These are called medullated nerve fibers, as distinguished 



FIG. 212. Spinal Ganglion, in longitudinal section, from Cat; the groups of nerve cells 

 lie embedded among the bundles of the nerve fibers. 



from the nonmedullated or those which lack the medul- 

 lary sheath. Fibers of the former kind are found in the 

 white substance of the brain and spinal cord, and run 

 to the muscles and organs of sense. Nonmedullated 

 fibers are found in the gray substance of the nervous 

 system. The axis cylinders are destitute of a sheath in 

 the neighborhood of the cell body. Scattered along the 

 fibers nuclei are found. The large nerve fibers may be 

 T2Vo of an inch in diameter, and some are supposed to 

 extend from cell bodies situated in the lower part of the 

 spinal cord down the leg to the foot. 



A bundle of nerve fibers surrounded by connective 

 tissue constitutes a nerve in the anatomical sense. 



