26 PHYSIOLOGY. 



these tissues are so arranged as to form a piece of animal 

 mechanism, designed for the performance of a certain office, 

 they form what is called an organ, as the lungs, brain, liver, 

 &c. The action of this organ is called its function. The 

 liver, for example, is an organ ; the conversion of the blood 

 which passes through it into bile, is its function. When 

 several organs are associated together for the accomplish- 

 ment of a common object, such an assemblage is called an 

 apparatus. The apparatus of digestion consists of the mouth, 

 teeth, esophagus, stomach, intestinal canal, liver, pancreas, 

 lacteals, &c., as all these concur in the process of digestion. 

 By system, is understood an assemblage of organs, possessing 

 a similar structure, as the nervous system, the muscular 

 system, &c. 



7. Membrane may be considered as the first or primary 

 tissue. It is the simplest form of organized substance, and 

 is extensively employed in the composition of the body. In- 

 deed, it is the principal material used in forming covering, 

 containing, protecting, and fixing every other component 

 part of it. It is this which contains in its cells the earthy 

 matter which goes to form the bones ; the canals in which 

 are deposited the substance which composes the muscular 

 and nervous tissues ; which forms a covering for the whole 

 body ; which lines all its internal surfaces ; surrounds all 

 its internal organs ; which makes up the solid portion of 

 every part of the system; forming the tubes and vessels, 

 such as the arteries and veins ; it connects all parts of the 

 body together, and fixes them in their several situations ; in 

 short, it is the substratum, or mould, in which all the other 

 particles are deposited, thus giving form and outline to the 

 whole body, so that if every other kind of animal matter 

 were removed, this tissue alone would preserve the exact 

 figure and present a perfect skeleton of the whole, and of every 

 one of its parts. 



8. There are several kinds of membrane ; the simplest form 

 of which, and that from which all the others arc supposed to 



