ORGANS j^ 



from hour to hour ; and a carnivorous animal has organs for 

 seizing and eating its food which are different from those of one 

 whose diet is vegetarian. This correspondence between organisa- 

 tion and mode of Hfe is known as adaptation. 



TISSUES 



Organisation involves more than the mere localising of functions 

 — more, that is, than the existence in the body of regions where 

 special functions are performed. It involves also a specialisation 

 of each of these regions to fit it for its special functions. This 

 speciahsation is found partly in the shape of each organ, but also 

 largely in its texture and composition. The substance of the body 

 is not alike throughout, but different portions of it have differences 

 in texture and chemical composition which confer upon them 

 different properties. Thus the outer layer of the skin is firm and 

 hard to penetrate, bone is rigid, blood is fluid, the substance 

 known as connective tissue is tough and binds other tissues 

 together, nerve has the power of conduction highly developed, 

 and muscle that of contraction, and so forth. Such a portion of 

 the body-substance with particular properties, due to a particular 

 texture and composition, is known as a tissue. An organ may 

 consist of one tissue throughout, but is usually built up of several, 

 upon the nature and arrangement of which its powers depend. 

 Thus a muscle contains, besides muscular tissue, connective 

 tissue to bind it together and nervous tissue to conduct through 

 it the impulses which cause it to contract. 



CO-ORDINATION 



Many of the processes which go on in living organisms lead to 

 action upon the outer world, but others are directed only to 

 keeping the machine in condition. The needs of the several 

 organs in the way of food, oxygen, and the removal of waste, 

 are very different, and vary from time to time with the activity 

 of the organ. Often, too, the activity of one organ must be 

 accompanied by an increase or depression of that of some other 

 organ, as when heavy work by muscles calls for a release by the 

 liver of fuel in the form of sugar, or in an active gland or muscle 

 the walls of the blood vessels relax their contraction and so 

 allow a better flow of blood through the working tissue. Again, in 



