32 ZOOLOGY 



animal's body to be dissolved away, a complete cast of each 

 in connective tissue would remain. When the ground sub- 

 stance of connective tissue in which fibres and cells lie 

 becomes impregnated with calcareous salts, it becomes changed 

 into what is called bone. 



Though "tissue" is a very suitable word to describe the 

 aggregation of cells and their products which constitute 

 the skeleton, it is by no means an accurate description of 

 aggregates of epithelial cells. Nevertheless it is applied in 

 zoology to denote all aggregates of similar cells, and so we speak of 

 epithelial, nervous, muscular, and glandular tissues. Epithelial 

 tisues consist of thin sheets of epithelial cells, in most cases 

 only one cell thick. In many cases it has been shown that 

 each epithelial cell is connected with its neighbours by bars 

 and bridges of cytoplasm. Where the "tissue" is more 

 than one layer thick it is usually found that only the inner- 

 most layer is in an actively growing healthy condition. The 

 other layers are in a decadent or dying state, and their 

 cytoplasm is being gradually converted into dead secretion. 

 A splendid example of this can be seen in the layers of cells 

 which make up the outer skin or " epidermis " of our bodies. 

 A moment's reflection will show us why this must be so. 

 All protoplasm, in order to be in vigorous life, must be in 

 immediate contact with a nutrient oxygen-carrying medium, 

 and when there are several layers of cells only one can be 

 so situated. 



Muscular tissue consists of bundles of muscle fibres which 

 in the vast majority of cases run parallel with one another, 

 so that the forces occasioned by their contraction act in the 

 same direction. Such a bundle is termed a muscle, it is 

 surrounded by a sheath of connective tissue termed a fascia, 

 and the individual fibres constituting the muscle are likewise 

 separated from one another by connective tissue which penetrates 

 between them. Only in the case of the muscular tissue which 

 makes up the heart of man and the higher animals has it 

 been ascertained that the muscular fibres are connected with 

 one another by bridges of protoplasm. 



Nervous tissue consists primarily of bunches or streaks 

 of nerve cells placed near each other, but not entering into 

 connection with one another. In many cases the axons of many 



