88 THE MECHANISM OF LIFE 



a certain average rate. But the organ is controlled by two sets 

 of nerves, some of which issue from the medulla via the tenth 

 (or vagus) cranial nerve, and are acceleratory ones, while others 

 come from the cervical sympathetic ganglia and are inhibitory 

 in their function (see p. 141). Increase of CO2 in the blood- 

 stream affects the nerve cells from which these fibres originate 

 in opposite ways; it tends to quicken the rate of impulses descend- 

 ing the sympathetic nerves, and so the heart's beat becomes 

 accelerated or quickened. More blood traverses the contracting 

 muscles, carrying nutritive matter and oxygen, and taking away 

 carbonic acid ; and at the same time more blood flows through the 

 capillaries in the lungs, so that more oxygen than usual is taken up 

 from the air in the lungs and more carbonic acid gas is excreted. 



There are other nervous centres which control the calibre of 

 the small arteries carrying blood to the skin. The walls of these 

 vessels contain muscle fibres which diminish the calibres of the 

 arteries, whereupon the latter contract, or they increase it, 

 and then the vessels relax. Two sets of nerve fibres, derived 

 from the sympathetic nervous system, go to the muscles of the 

 cutaneous bloodvessels, one set, which is called vaso-constrictor 

 (constricting the arteries), and another set called vaso-dilator 

 (dilating them). The effect of the sudden generation of heat 

 and carbonic acid is to set up reactions in the central nervous 

 system which lead to the issue of nervous impulses, ma the 

 sympathetic ganglia, to the cutaneous bloodvessels, causing the 

 latter to dilate. More blood then passes through the skin than 

 usual, the latter flushes, and the sweat glands become more 

 active than is normally the case. The skin becomes moist, there 

 is evaporation of water from its surface, and so the heat generated 

 by the muscular activity is eliminated. 



Here we have a series of regulations carried out by the nervous 

 system. One set of organs, the muscles of the trunk and limbs, 

 have been functioning at a greater rate than usual, and the effect 

 of this alone would be to disturb the general balance of bodily 

 activity. Heart, respiratory organs, and skin are therefore 

 stimulated by the central nervous system also to function more 

 vigorously than usual, so that the increased activity of the 

 muscular organs is compensated. 



The nervous system is therefore the mechanism of regulation, 

 co-ordination, and integration of bodily activities, and it is from 

 this point of view that we study it. But first of ^,11 we jnuist 



