90 HUMAN PHYSIOLOGY 



exchanged for that of breathing out. Here we come 

 face to face with an act which is largely mechanical 

 in its nature, and does not involve muscular action, 

 In breathing out we simply see the recoil of the chest, 

 and the return of the elastic structures to their posi- 

 tion of rest from which they were disturbed by the 

 act of breathing in. Inspiration, therefore, is the 

 one act which involves an expenditure of muscular 

 energy, while breathing out as a mechanical act 

 makes no demands upon our working power. Nature 

 in this respect supplies an example which might be 

 paralleled by many other instances, of an economical 

 use of her working powers. 



THE SKIN. The lungs do not stand alone in the 

 category of our bodily belongings. As a matter of 

 fact they represent one of a number of organs per- 

 forming the same work although the labour is carried 

 out in different fashions. The neighbour organs of 

 the lungs are the skin and kidneys, so that the three 

 form a kind of physiological trio whose action is that 

 of excreting waste matter from the body, the lungs 

 acting in a highly important fashion because they like- 

 wise perform the function of absorbing oxygen. The 

 functions of the skin are of a varied character. In 

 addition to serving as a body-covering, the nerves of 

 its under layer supply us with the means of exercis- 

 ing the sense of touch. The glands of the skin con- 

 stitute it an organ of excretion for the getting rid of 

 waste matter, whilst the large supply of blood con- 

 tained in these minute bloodvessels renders the skin 

 an organ which regulates to a large degree the tem- 

 perature of the body. These varied functions being 

 borne in mind teach us that the skin is really an ex- 

 tremely complex structure, and in respect of its func- 

 tions it may be described legitimately as a kind of lung 



