86 



HUMAN TISSUES 



Ibiceps 



.triceps 



niiM 



Muscles work in pairs. Motion is caused by the pull 

 resulting from muscular contraction. If the biceps con- 

 tracts, the triceps relaxes; the forearm is thus pulled up. 



keeps the muscles in a condition ready for work. Muscles always 

 exert a pull, not a push. Skeletal muscles usually occur in pairs, 

 one of which opposes the other. For example, in front of the 



upper arm there is a 

 muscle called the flexor 

 and on the o})posite 

 side, the antagonizing 

 muscle called the ex- 

 tensor. The former 

 causes the forearm to 

 bend and the latter 

 causes it to extend. 



Nerve tissue. The 

 organs of the body are 

 composed of various tissues. These tissues and organs are inter- 

 related and brought into communication by means of the nerve 

 tissue. As telephone wires bring various homes of a community 

 into communication, so nerve tissue brings the various organs 

 into coordination. For example, we see a coin on the floor and 

 pick it up. The eyes in seeing, the mind in deciding, the body 

 in bending, and the fingers in picking up, all work in proper 

 sequence due to regulation by the nerves. The muscles actu- 

 ally do the w^ork but the nerves control 

 the muscles. Muscles are kept in proper 

 tone by repeated stimulation from the nerve 

 cells. 



If a nerve cell is examined microscopi- 

 cally, the cell body, cyton, with branching 

 projections of protoplasm and with one long 

 process is easily seen. The branching pro- 

 jections are the dendrites and the one long 



A nerve is a bundle o^ 



process is the axon. The nucleus lies near axons. Each bundle is sur- 



nil T» i? rounded by a fatty protet- 



the center oi the ceil bodv. Bv means or tive sheath. 



