THE RESPIRATORY SYSTEM. 



275 



(2) Impulses from the Higher Parts of the Brain. In an anaesthe- 

 tised animal, the brain stem may be divided in the pons or upper part 

 of the medulla without any obvious change being produced in the 

 respiratory movements. Nevertheless, impulses from the higher parts 

 of the brain can greatly modify respiration, and the effect on respiration 

 of emotional states, such as anger or excitement, is often very marked. 



Again, the respiratory movements become deeper and more frequent 

 at the very beginning of severe muscular exercise ; even the first breath 

 taken after the onset of muscular work is much deeper than the respira- 

 tion during rest (fig. 110). These changes occur too quickly to be 



FIG. 110. Effect of muscular work on the respiratory movements. (Krogh.) 

 The work begins at X. Tracing to be readjjfrom right to left. 



brought about by an increase in the tension of carbonic acid in the 

 blood, and recent observation shows that they are due to impulses 

 passing from the cerebral cortex to the respiratory centre, which render 

 it more sensitive than before to the presence of carbonic acid in the 

 blood. Hence the normal tension of carbonic acid, acting on the 

 unusually sensitive centre, calls forth deeper and more rapid respirations. 

 By this means the amount of oxygen entering the lungs and passing to 

 the blood and tissues is increased at the very beginning of exercise ; and 

 the muscles are able to take up from the blood, without delay, the 

 additional oxygen which they need for their increased activity. 



(3) The respiratory movements may also be modified by impulses 

 reaching the centre from almost every region of the body. Thus pain- 

 ful stimuli usually produce hyperpncea, whereas impulses passing along 

 the fifth nerve and the nerves from the upper respiratory passages 

 tend to inhibit respiration ; these nerves may be stimulated by irritant 

 vapours, such as that of ammonia. The superior laryngeal nerve 

 supplies sensory fibres to the glottis ; and stimulation of its endings, 

 for instance, by the entrance of a crumb into the glottis, inhibits in- 

 spiration and causes violent expiratory efforts. Electrical stimulation 

 of the central end of the nerve brings about the same effect. 



