28 *THE HUMAN BODY 



the organism, and upon them in great degree the perpetuation of 

 the race depends. Their relationships are less familiar than those 

 of the immediately adaptive functions, but as we study them, in 

 due course, their fundamental importance will become clear. 



Co-ordination in the Body. A very little study of our most 

 common activities shows that in us the function of co-ordination 

 is developed to a very high degree. In the act of walking, for 

 example, sensations of sight direct the movements of the muscles 

 of the legs. To cause the leg muscles to work adaptively in 

 obedience to the sensations entering the eyes a special phase of 

 co-ordination, namely, conduction of messages from one point to 

 another, enters prominently. This form of conduction is accom- 

 plished through the operation of the nervous system, and the kind 

 of co-ordination of which it is a part is called nervous co-ordination. 

 There is another sort of co-ordination which is very important, 

 and of which we shall have much to say, but which is in many 

 respects less familiar in its workings. In the growth process, for 

 instance, co-ordination enters constantly. In most individuals 

 the two legs are the same length, so are the two arms; the ears are 

 about the same size; the eyes are the same color. These things 

 do not happen by accident, but because they are controlled by a 

 definite co-ordinating mechanism. This type of co-ordination 

 differs from that effected through the nervous system chiefly in 

 that it is concerned with processes which go on more slowly. The 

 body carries on this type of co-ordination by means of chemical 

 substances, known as hormones, which are manufactured in certain 

 tissues of the body, specially differentiated for that purpose, and 

 conveyed to the tissues upon which they exert their influence 

 through the blood stream. This method of control is called 

 chemical co-ordination and shares with nervous co-ordination the 

 task of causing the different parts of the exceedingly complex body 

 machine to operate harmoniously. 



Emphasis is placed upon the property of co-ordination thus 

 early in our consideration of the Body because a true appreciation 

 of Physiology requires not only an understanding of the working 

 of the various tissues, but even more a grasp of the manner in 

 which they co-operate to secure that continuous adaptation of 

 the organism to the environment upon which life depends. 



