12 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



their molecules, so the physiological peculiarities of organized bodies 

 are dependent on their cellular structure. 



Physiology is therefore the study of the properties of cells. Cells 

 possess the properties of Nutrition, Reproduction, Growth, Develop- 

 ment, and in many cases their contents are capable of Motion and mani- 

 festing Irritability. 



I. THE GENERAL PROPERTIES OF CELLS. 



Microscopic examination teaches that every living object, from man 

 down to the smallest animalcule invisible to the naked eye, from the 

 largest tree down to the most microscopic plant, is built up on the 

 same general plan. In each the same element of organization is found, 

 and every living form is built up of associations of these microscopic 

 units, each of which, even in the most complex forms of life, may be 

 regarded as separate individual organisms.* 



For even in complex organisms cells to a certain extent carry on a 

 separate and independent existence. We see 

 separate cells originate separately, grow, repro- 

 duce themselves, become diseased and die 

 without the entire organism as a whole taking 

 any part in these different stages of existence 

 of its component parts. The individual life of 

 each separate cell is recognized in the different 

 activities of different cells : the activity of the 

 FIG i TYPICAL ANIMAL organism is the result of the, sum of these 

 ( mn V ) VUM F separate existences. 



In tlieir typical form both animal and 

 vegetable cells consist of closed vesicles, with 



homogeneous or striated walls, a viscid albuminous contents, termed 

 protoplasm, containing an aggregation of granules called -a nucleus, 

 within which again is a still denser formation called a nucleolus. The 

 cell-contents is frequently vacuolated, i.e., contains minute cavities filled 

 with a clear fluid. 



The contents of the cells, which we shall find to be functionally the 

 most important, is called protoplasm. It is a transparent mass in which 

 numerous granules are suspended, and which possesses in all young cells 

 (the property of contractility. It is often seen to be reticulated. In 

 older cells the quantity of fluid diminishes and the cells become firmer 

 and drier, while vacuoles often form and contain fluid. This change in 

 .the physical properties of cells is often associated with a visible change 

 in their chemical nature, thus, with a deposit of coloring matter, starch 



* Such units of organization are termed cells, from the resemblance which micro- 

 scopic sections of young tissues, whether plant or animal, bear to a honey-comb. 



