Chap. Ill] ORGAN 25 



(2) The simplest form of cell is spherical, but this is seldom realized 

 except in unicellular plants and animals. In the human body the 

 form of the cell is modified by the pressure of the surrounding 

 structures, by active movements of the cell substance, and by 

 growth and differentiation. (3) It is assumed that the marked 

 difference in the appearance of cells is an expression of a chemical 

 difference, which in turn shows the difference in function. (4) A 

 unicellular animal is in itself a complete living thing, and thus 

 one cell must perform all the essential activities of life, and is self- 

 sufficient. In the human animal the individual cells have become 

 specialized as it were, and certain groups of them perform certain 

 functions, i.e. the function of muscle cells is to contract, and the 

 combined contraction of a group of muscles cells results in the 

 contraction of a muscle. 



TISSUE 



A collection of cells of like substance arranged together form 

 what is known as a tissue. In many tissues, all the substance is 

 not inside the cell walls, some of it is between the cells or inter- 

 cellular. In the muscles there is a cement substance between the 

 cells which holds them together. In some tissues there is very 

 little intercellular substance, in others there is a large proportion 

 of it. 



ORGAN 



When two or more different tissues are associated in per- 

 forming some special office in the body, the part so adapted is 

 termed an organ. Thus, the lungs are organs specially adapted 

 for assisting in the function of respiration, the bones are organs 

 adapted for support and locomotion, the kidneys for secreting 

 urine, etc. As the structure of an organ depends upon the prop- 

 erties of the tissues composing it, so the characteristics of each 

 tissue depend upon their ultimate structural units — the cells 

 and the intercellular substance. 



of simplicity in converting figures in the text, from one system to the other, we 

 have assumed 



1 cm. to equal | in. (25 mm. = 1 in.). 



1 cc. to equal 15 minims. 



30 gm. to equal 1 oz. (dry or liquid measure). 



30 cc. to equal 1 oz. (dry or liquid measure). 



1 litre to equal 1.75 pt. (dry measure). 



1 litre to equal 2.11 pt. (liquid measure). 



