556 TEXT-BOOK OF PHYSIOLOGY. 



glottis is kept widely open for respiratory purposes by the tonic con- 

 traction of the abductor muscles (the crico-arytenoids) ; for phonatory 

 purposes the glottis is closed and the vocal membranes approximated 

 by the contraction of the adductor muscles. It has been shown that 

 these opposed groups of muscles have independent nerve-supplies; 

 that two sets of fibers in the common trunk can be separated and 

 stimulated independently of each other. Feeble stimulation of the 

 common trunk produces a still further abduction of the vocal cords. 

 With an increase in the strength of the stimulus, however, the reverse 

 obtains: namely, adduction which increases until the glottis is com- 

 pletely closed. Division of the nerves is followed by paralysis of both 

 the phonatory and respiratory muscles, the abductors and adductors, 

 with the result of seriously impairing both phonation and respiration 

 and not infrequently causing death. The fibers of the inferior laryn- 

 geal nerve are derived from the eleventh nerve, the spinal accessory. 



The Cardiac Nerves. Faradization of the trunk of the vagus or 

 of the peripheral end of the divided nerve gives rise to a diminution 

 in the frequency and force of the heart's contractions ; and if the stim- 

 ulation be sufficiently powerful, completely arrests it in the phase of 

 diastole. To these results the term inhibition is applied. Division 

 of the vagi or of the cardiac branches is followed by an increase in 

 the number of the contractions from loss of inhibitor influences. 

 The inhibitor fibers of the vagus are generally believed to be derived 

 from the spinal accessory, though this has been questioned. Accord- 

 ing to the recent investigations of Schaternikoff and Friedenthal, 

 they come direct in the vagus, from a nucleus near the vagal motor 

 nucleus in the medulla, the spinal accessory sending no branches to 

 the heart. In the frog and other batrachia the vagus contains also 

 accelerator or augmentor fibers derived from the sympathetic ; hence 

 stimulation, especially if feeble, may increase the heart's action or 

 may only retard, but not arrest, the heart. 



The Pulmonary Nerves. The pulmonary nerves, given off from 

 the trunk after its entrance into the thorax, do not lend themselves 

 readily to experimentation. Division of both vagi in the neck above 

 the point of exit of the pulmonary branches is followed by a decrease 

 in the frequency of the respiratory acts, with an increase in their depth. 

 At the same time there is a loss of sensibility of the mucous membrane 

 of the trachea and lungs and a paralysis of non-striated muscle-fibers. 

 Stimulation of the central end of the vagus increases the frequency, 

 but decreases the amplitude, of the respiratory movements. If the 

 stimulation be increased in intensity the respiratory movements in- 

 crease in frequency until the inspiratory muscles pass into the con- 

 dition of tetanus. 



Feeble stimulation of the vagus not infrequently inhibits the 

 inspiratory movement and increases the expiratory until there is a 



