568 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



nary circumstances, the quantities of O and CO 2 in the blood, and the impulses 

 conveyed from the lungs by the fibres of the pneumogastric nerves. 



The Afferent Respiratory Nerves. The chief of these nerves are the 

 pneumogastric, glosso-pharyngeal, trigeminal, and cutaneous nerves. The im- 

 portant part taken by them in the regulation of the respiratory movements has 

 frequently been alluded to in connection with the respiratory centres. Their 

 functions, however, are of sufficient importance to demand special and detailed 

 consideration. 



The pneumogastric nerves are pre-eminently the most important. Their 

 functions may be studied by comparing the phenomena before and after section 

 of one or of both nerves, and from the results following excitation by stimuli 

 of varying quality and strength under normal and abnormal conditions. 



Section of one pneumogastric may be without effect or be followed by a 

 transitory, slight diminution of the respiratory rate; by slower and deeper 

 movements; by stronger, deeper, and longer inspirations; by unaltered or 

 longer or shorter expirations; and probably by active expirations. These 

 effects are transient, and the normal respiratory movements are usually restored 

 within a half hour. Section of both nerves is sooner or later followed by a 

 diminution of the respiratory rate ; by slow, deep, powerful inspirations ; by 

 active expiration ; and by a pause between expiration and inspiration. The 

 immediate results are variable unless certain precautions are taken to prevent 

 irritation of the central ends of the cut nerves. If the ends are allowed to 

 fall back into the wound, the respirations may become irregular; or they may 

 be less frequent, with weakened inspirations, spasmodic expirations, and pro- 

 longed expiratory pauses. The explanation of these variable results is found 

 in the fact that the expiratory fibres are more sensitive to very weak stimulus 

 than the inspiratory fibres, and that the mechanical irritation caused by the 

 section, and the excitation due to the electric current in the cut ends of the 

 nerves that is established when the central end of the nerve is replaced in the 

 wound, excite expiratory impulses and cause expiratory phenomena; if the 

 irritation be stronger, both inspiratory and expiratory impulses are excited, 

 thus causing uncertain results, varying as one or the other is the stronger. If 

 irritation be prevented, section is at once followed by typical slow, deep 

 respirations. 



Stimulation of the central end, of the cut vagus, the other nerve being 

 intact, is followed by variable results dependent upon the character of the 

 stimulus. Chemical stimuli, such as a solution of sodium carbonate, excite 

 the expiratory fibres; mechanical stimuli, the inspiratory fibres; electrical 

 stimuli, expiratory or inspiratory fibres or both, according to the strength of 

 the current. Single induction shocks are without effect, but a tetanizing 

 current is very effective. Should that current which will elicit the least 

 response be used, the breathing is rendered less frequent, the inspirations are 

 weakened, and the expirations may be active and lengthened ; in other words, 

 there are present the same phenomena which often immediately follow section 

 of both nerves when the cut ends are allowed to fall back into the wound and 



