20 MANUAL OF ELEMENTARY ZOOLOGY 



together, 1 nerve has the power of conduction highly de- 

 veloped, and muscle that of contraction, and so forth. 

 Such a portion of the body-substance with particular 

 properties, due to a particular texture and composition, is 

 known as a tissue. An organ may consist of one tissue 

 throughout, but is usually built up of several, upon the 

 nature and arrangement of which its powers depend. Thus 

 a muscle contains, besides muscular tissue, connective 

 tissue to bind it together and nervous tissue to conduct 

 through it the impulses which cause it to contract. 



We have hitherto spoken of the body as though it were 



alive throughout. This, however, is rarely the 

 ro op asm. casSi ^g ii v j n g p ar t of the body of all animals 

 is a soft, slimy substance known as protoplasm. In a few 

 cases this forms the whole body, but in most it is only a 

 part. All tissues contain protoplasm, but many contain also 

 a framework of other substances known as formed material, 

 made and secreted by the protoplasm and serving for its 

 support. Thus in bone (Fig. 4) there is a groundwork, 

 consisting largely of salts of lime, to which it owes its 

 hardness, and this groundwork is penetrated by a mesh- 

 work of protoplasm. In composition, protoplasm is a 

 solution in water of organic substances and salts, especially 

 characterised by the presence of proteins. In many cases, 

 as we shall see in a later chapter, the protoplasm is divided 

 into minute units known as cells (Fig. 5). 2 



The study of living organisms is known as Biology, and 



is divided into Botany, which deals with plants, 

 Physiology!" 1 an< ^ Zoology, which deals with animals. Now 



an organism may be regarded from two points 

 of view according as attention is concentrated upon its 

 structure or its functions, though of course the connection 

 of structure with function makes it impossible to study 

 either without reference to the other. The sciences of 

 Zoology and Botany are correspondingly divided each 

 into two subordinate sciences, Anatomy or Morphology, 

 which deals with the structure of the bodies of organisms, and 



1 This may be seen in skinning any large animal. The tough, white 

 material which holds down the skin and binds the muscles together is 

 connective tissue. 



2 For the definition of this term see p. 84. 



