242 PRACTICAL ZOOLOGY 



(i) Movements are performed by muscular tissue. This 

 tissue is made up of muscle cells, either voluntary or involun- 

 tary. The voluntary muscle cells form muscles that are con- 

 trolled by the will of the animal; they are cross-striated (Fig. 

 140, E). Involuntary muscles cannot be controlled; they are 

 smooth and non-striated (Fig. 140, F). 



(2) The perception of changes in the surroundings and the 

 conduction of impulses are functions of nervous tissue. Nerve 

 cells are peculiar structures consisting of a nucleated central 

 body, from which branches and nerve fibers extend (Fig. 140, G). 

 These nerve fibers may be several feet in length, extending, for 

 example, from the lower part of the backbone in man to the toes. 



(3) The various parts of the body are bound together by 

 connective tissues, such as tendons, and held upright and pro- 

 tected by supporting tissues, such as bone and cartilage (Fig. 

 140, D). The substances in these tissues are largely of non- 

 living fibers, plates, and masses produced by living cells. 



(4) The body surface and the surfaces and linings of organs 

 are composed of epithelial tissues. The epithelial tissues cover- 

 ing the body serve as a protection, and contain nerve endings, 

 glands, hairs, etc. (Fig. 140, C). 



Living and Lifeless Things. — Living things differ from life- 

 less things in being 



(1) of definite size, and not of any size like water which may 

 exist as a particle of vapor or as an ocean ; 



(2) of definite form, and not of varied form like water, 



(3) of definite organization into cells ; 



(4) capable of growth by the addition of new particles among 

 preexisting particles ; 



(5) able to reproduce others of their kind; and 



(6) able to move. The movement of a thing is usually 

 enough to indicate that it is alive, but many living things, such 

 as a hen's egg, are unable to move ; hence in many cases we must 

 consider the other five characteristics first listed in order to de- 

 termine whether a thing is alive or not. 



