20 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



pared to battery cells generating, the other to the wires 

 transmitting, electric energy. The one is found only in 

 the nerve centers, such as the brain, the spinal cord, etc. ; 



the other also and most char- 

 acteristically in the nerves 

 proper. We will discuss this 

 more fully in connection with 

 the physiology of the nervous 

 system. 



6. Epithelial Tissue.— 

 The whole surface of the ani- 

 mal body, both external and 

 internal, is covered with a 

 pavement of living nucleated 

 cells. These are called epithe- 

 lial cells. In the higher ver- 

 tebrates those on the outer 

 surface dry up, become more 

 or less indurated, and are 

 called epidermal. Those on 

 the interior surfaces or mu- 



FiG. ii.— Vertical section of a COUS membranes always re- 

 convolution of the brain, show- • fh ■ r f art .: nonfil- 

 ing the cells of the gray gran- tau ,K!l sou, active con,. 



tion. They are constantly 



ular matter (gg ) giving out 

 fibers which go to form the 



white fibrous matter (w / ) be- dying, dissolving and passing 



low. After Luys. r , " 



away as mucus of the mucous 

 surfaces, and as constantly being born and the pavement 

 renewed. It is the least modified of all the tissues. 



Function. — Their function is perhaps the most impor- 

 tant in the body, viz., the absorption of matter (food) 

 from the external world into the interior of the body, 

 and the elimination of waste matter from the body into 

 the external world. In other words, the whole exchange 

 of matter between the exterior and interior is carried on 

 through this tissue. 



