46 



STBUCTUEAL UNITS OF HUMAJJT TISSUES 



ening being the sum of the shortenings of the respective 

 cells. 



Nerve tissue. — In every human being there are two 

 organs called the brain and the spinal cord. From these 

 there extends all over the body a lacework of fibers which 

 are called nerves. Brain, cord, and nerves.are built of the 



substance called nerve tissue, 

 whose principal purpose 

 seems to be to originate and 

 conduct stimulations to vari- 

 ous parts of the body. These 

 stimuli act upon the other 

 tissues of the body and cause 

 them to either increase, or 

 cease their special kind of 

 activity. In this way is con- 

 trolled the action of the dif- 

 ferent organs that are made 

 up of tissues. Nervous tissue 

 is made up of cells just as are 

 aU other tissues. These cells 

 are, however, peculiarly ar- 

 ranged and extended so as to 

 be continually in contact. For 

 example, while the muscles of 

 the foot are entirely distinct 

 and separated from those of the hand, the nerve cells of 

 the hand are, on the contrary, connected with those of the 

 foot and other parts of the body, in the same way that tele- 

 graph stations are connected. In other words, if we could 

 take away all the other tissues of the body we should have 

 left the nervous tissue in the form of an intricate network, 



Fig. 18 — Nerve cell, showing nucleus 

 in the center of the cell, andhaving 

 one axis cylinder or nerve process 

 and several dendrites. 



