A STUDY OF THE NERVOUS SYSTEM 263 



these nerve cells, however, have two long processes : one 

 comes in along the nerve 

 trunk from the organs of 

 the body, the other runs 

 from the ganglion into the 

 dorsal part of the cord, 

 ending at length in a ter- / 

 minal brush like those al- 

 ready described (Fig. 117). 

 Relation of Cells and 

 Fibers. The nervous sys- 

 tem, then, is made up of a 

 very great number of dis- FlG - 122. -Cross Section of Nerve 



tinct units, called nerve c 



. Snowing smaller bundles ot nerve fibers 



cells. The I'OOtlike processes surrounded by connective tissue, 

 that reach out from the Magnified 6 times. Photographed 

 , , , , . iii -> through the microscope. 



cell bodies probably aid 



in bringing about cooperation between the various cells in 

 a nerve center. And finally, the long axis cylinder which 

 extends from each nerve cell serves like a telegraph wire to 

 connect the distant muscle or skin with the central nerve 

 station. The length of these slender axis cylinders some- 

 times measures several feet, as is the case with those which 

 run from the spinal cord to the tips of the toes 



3. PHYSIOLOGY OF THE SPINAL CORD AND SPINAL NERVES 



Experiments on Animals. The functions of various parts 

 of the nervous system have been determined to a large ex- 

 tent by experiments performed on animals. When a dog, 

 for instance, is given ether, it is made insensible to pain, 

 and the large nerve trunks that supply one of the front legs 

 may then be severed near the shoulder. On recovering from 

 the effects of the ether, the animal is found to have lost all 

 sensation and all power of movement in this leg. But when 

 the cut ends of the nerves are brought into contact and the 



