16 APPLIED PHYSIOLOGY 



12. Organs. In order to work to the best advantage, 

 several kinds of tissues are usually associated together. 

 Thus, the stomach, which digests food, is composed of 

 muscular tissue which moves the food about, and epithelial 

 tissue which pours out digestive fluids, and nervous tissue 

 which presides over the process, while connective tissue 

 binds the whole together. A collection of different tis- 

 sues always arranged in a definite, compact shape for a 

 special purpose, is called an organ. The stomach, intestine, 

 pancreas, and the liver are the four principal organs of 

 digestion. The lungs are organs of respiration, the heart 

 is an organ for the propulsion of blood. The kidneys and 

 skin are organs which get rid of waste matter, and the 

 brain is the organ of thought. The term organ is also 

 applied to many other parts of the body, but these are 

 the principal ones. 



13. Systems. Sometimes a definite series of tissues 

 and organs are not arranged in compact form, but are 

 scattered through the whole body. This forms a system. 

 Thus the system of tubes formed of muscular and con- 

 nective tissue in which the blood moves is called the 

 circulatory system, while the heart is an organ in the cir- 

 culatory system. In the same way the brain is an organ 

 in the nervous system. The five main systems are the 

 digestive, circulatory, respiratory, nervous, and excretory 

 systems. In Physiology the action of the cells of each 

 tissue, organ, and system is studied separately. The 

 structure and arrangement of the cells of each tissue 

 are studied by means of a microscope. 



14. The microscope. In order to show even the largest 

 cell, a compound microscope magnifying at least twenty 

 times is needed ; while for ordinary use, one magnifying 

 at least two hundred times is necessary. 



