IV THE NERVES OF RESPIRATION 157 



As the nasal passages cannot be closed by their own 

 action, air has always free access to the pharynx ; but the 

 glottis, or entrance to the windpipe, is completely under 

 the control of the nervous system — the smallest irritation 

 about the mucous membrane in its neighbourhood being 

 conveyed, by its nerves, to that part of the cerebro-spinal 

 axis which is called the spinal bulb or medulla ob- 

 longata (see Lesson XI. ). The spinal bulb thus stimu- 

 lated gives rise, by a process which will be explained 

 hereafter, termed reflex action, to the contraction of the 

 muscles which close the glottis, and commonly, at the 

 same time, to a violent contraction of the expiratory 

 muscles, producing a cough (see p. 146). The muscular 

 fibres of the smaller bronchial tubes are similarly under the 

 control of the bulb, sometimes contracting so as to narrow 

 and sometimes relaxing so as to permit the widening of 

 the bronchial passages. 



These, however, are mere incidental actions. The whole 

 respiratory machinery is worked by a nervous apparatus. 

 From what has been said, it is obvious that there are 

 many analogies between the circulatory and the respiratory 

 apparatus. Each consists, essentially, of a kind of pump 

 which distributes a fluid (liquid in the one case, gaseous 

 in the other) through a series of ramified distributing 

 tubes to a system of cavities (capillaries or air-cells), the 

 volume of the contents of which is greater than that of 

 the tubes. While the heart however is a force-pump, the 

 respiratory machinery represents ^i suction-pump. 



In each the pump is the cause of the motion of the 

 fluid, though that motion may be regulated, locally, by 

 the contraction or relaxation, of the muscular fibres 

 contained in the walls of the distributing tubes. But, 

 while the rhythmic movement of the heart chiefly depends 

 upon an apparatus placed within itself, which is then con- 

 trolled by the central nervous system, that of the respi- 

 ratory apparatus results mainly from the operation of a 



