134 RESPIRATION 



and spinal nerves and also by intraeentral paths, passing 

 from the cerebrum to the medulla. The effect of sensory im- 

 pulses on the respiratory centre is varied. In general, however, 

 stimulation of cutaneous nerves gives one of two effects; either 

 a stimulating action, shown by more active inspirations and 

 expirations, or an inhibitory effect in which respirations 

 become slower and feebler and may cease altogether. It is 

 customary, therefore, to speak of respiratory pressor and de+ 

 pressor fibers. The rhythm of respiration may be affected 

 by the will and by the emotions, by the quality, and the tem- 

 perature of the blood, and by afferent impulses, coming 

 particularly over the tenth nerve. Section of the latter on 

 one side may slow and deepen respiration while section of 

 both vagi almost invariably exaggerates this effect leading 

 to distinct pauses between the respiratory movements. When 

 the central end of the cut vagus is stimulated, respiration is 

 affected in a variety of ways depending upon the strength 

 of the stimulus and the condition of the centre. With weak 

 stimuli the inspiratory movements are inhibited partially 

 or completely, leading to a cessation of respiration with the 

 thorax in the stage of passive expiration. Or, the rate of the 

 inspiratory movements may be increased and thus lead to 

 cessation of respiration with the thorax in an inspiratory 

 position. These effects are interpreted as due to two sets 

 of fibers: (1) Inspiratory fibers whose effect is to quicken the 

 rate of discharge of the respiratory centre, and (2) inspiratory 

 inhibiting fibers whose effect is to inhibit, partially or com- 

 pletely, the discharges of the respiratory centre. Excitation 

 of the superior laryngeal fibers always inhibits respiration 

 the chest coming to rest in the position of passive expiration. 

 Excitation of the glossopharyngeal nerves has a similar effect, 

 but the inhibitory influence lasts for three or four successive 

 respiratory acts. Irritating gases affect the trigeminal nerve 

 endings in the nose or the endings of the tenth nerve in the 

 larynx and lungs. The effect of the excitation of the cutaneous 

 nerves may be seen in a cold douche, which primarily increases 

 the respiratory rate and may cause its cessation. 



The efferent respiratory nerves are the phrenics, which 

 supply the diaphragm, certain spinal and cranial nerves supply- 

 ing respiratory muscles. Section of one phrenic causes paralysis 



