402 THE NEEVOUS SYSTEM 



spinal nerves. The places where these roots emerge 

 appear as four grooves on the surface of the cord. The 

 posterior roots show a swelling just back of the point where 

 they unite with the anterior fibers. This swelling is 

 called the spinal ganglion. Experiment has shown that 

 the anterior roots are composed mainly of efferent or 

 motor fibers and convey motor impulses to the muscles, 

 while the fibers of the posterior root are afferent or sensory 

 and bring sensory impulses to the cord. The spinal 

 nerves which contain both kinds of fibers are, therefore, 

 mixed nerves. 



Structure of a spinal nerve trunk and fiber. — If we cut 

 a cross section of a spinal nerve we find it made up of 

 many bundles of nerve fibers. The entire trunk is sur- 

 rounded with a sheath of connective tissue. Inside this 

 sheath, each bundle of fibers is wrapped in a sheath of con- 

 nective tissue called the perineurium. We may remove 

 this sheath and separate the bundles into the nerve fibers 

 themselves. 



In these ultimate nerve fibers, thus separated, we have 

 the actual conductors of impulses, and our dissection shows 

 that the spinal nerve may aptly be compared to the cables 

 of a telephone station where each cable consists of bundles 

 of separate wires, both bundles and wires being care- 

 fully insulated from one another. 



Examination of a single nerve fiber shows it also to be 

 composed of layers. In the middle is a core, or axis, com- 

 posed of modified protoplasm. This is called the axis cy- 

 linder, and it is this cylinder which transmits the impulse. 

 Surrounding this is a sheath called the medullary or myelin 

 sheath. This sheath varies greatly in thickness in different 

 fibers and is often segmented, forming the so-called " nodes 



