NERVE CELLS 



47 



all parts of which were connected. It is this connection 

 that allows us to become conscious of an itching sensation 

 in the foot, and permits us to direct the muscles of the hand 

 to rub the affected part. The manner of the connection of 

 these cells is peculiar to this tissue, and its method we shall 

 investigate further in a later chapter. 



Nerve cells. — As shown under the 

 microscope, a nerve cell consists (see 

 Ex, XXVIII. and Fig. 18) of a mass of 

 protoplasm with a nucleus. The proto- 

 plasm is drawn out into two forms of 

 projections, nerve processes and den- 

 drites. The rodlike nerve processes 

 (axis cylinders) form the center of a 

 fiber called a nerve and this latter may 

 be of great length. These nerves may 

 branch extensively, and through its 

 nerve process permit a nerve ceU located 

 in the brain or cord to be in contact 

 with widely distant points, say in the 

 finger or foot. By means of the den- 

 drites, one cell and its nerve may be 

 brought in contact with another cell and 

 nerve, and thus the entire set of cells 

 brought into communication. A nerve 

 ceU with its processes and dendrites is 

 called a neuron. (See Fig. 19.) Just 

 as the protoplasm of a muscle ceU has 

 the power of expansion and contraction 

 developed to a great degree, so the pro- 

 toplasm of a 'nerve cell has developed the power of con- 

 ducting impulses of various kinds. 



Fig. 19 — Neuron; np, 

 nerve process; n, 

 nerve cell ; t, ter- 

 minal brush ; o, d, 

 nerve fiber with 

 sbeatbs surrounding 

 the axis cylinder. 



