CHAPTER V 



The cell theory as illustrated by the histological structure of the higher 

 animals and plants Limitations of the cell theory The cell as the 

 physiological unit. 



IN all the higher animals and plants the constituent cells of 

 the adult body are grouped in more or less well defined tissues, 

 which originate from the fertilized ovum in the manner indicated 

 in the last chapter, and the cells of each tissue co-operate 

 with one another in the fulfilment of some common function. 

 The study of the microscopic structure of tissues is termed 

 histology. 



As examples of animal tissues we may take blood, epithelium, 

 fat, cartilage, muscular tissue and nervous tissue, as met with 

 in typical vertebrates. 



Blood is exceptional in that it is a liquid tissue, a condition 

 which is of course necessary in order that it may circulate through 

 the blood-vessels and perform its functions as -the distributor 

 throughout the body of food material and oxygen, arid the carrier 

 of carbon dioxide and other waste products from all the various 

 parts of the body to the special organs of respiration and excre- 

 tion. Floating in the liqui^jsortion, or plasma, are found two 

 kinds of cells, the' white and- fed blood-corpuscles. 



The white corpuscles, or leucocytes (Fig. 15, a.), closely 

 resemble Amoeba3. They are colourless, nucleated cells, exhibiting 

 amoeboid movements, and they have the remarkable power of 

 creeping through the thin walls of the blood-capillaries into the 

 surrounding tissues. Like the Amoeba they feed, in part at any 

 rate, by taking in and digesting the bodies of other minute 

 organisms, and they grow and multiply by simple fission. They 

 exhibit a much greater degree of independence than most of the 

 cells of the body and can even live outside the body for a time if 

 kept in suitable culture media and at the proper temperature. 

 Their most important function appears to be to defend the body 

 from the attacks of bacteria and other harmful micro-organisms. 



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