6 S2 TEXT-BOOK OF PHYSIOLOGY 



the pitch, and a diminution in the clearness of the voice. In consequence 

 of the loss of the sensibility there is an inability to perceive the entrance of 

 foreign bodies into the larynx. 



The Depressor Nerve. Stimulation of the peripheral end of the depres- 

 sor nerve is without effect; stimulation of the central end retards and even 

 arrests the heart's pulsations and lowers the general blood-pressure. These 

 two effects, though associated, are nevertheless independent of each other. If 

 the vagus nerves be divided on both sides between the origin of the depres- 

 sor and the origin of the cardiac nerves, and the former stimulated, there 

 will be a fall of pressure without retardation of the heart. The effect on the 

 heart is attributed to a stimulation of the cardio-inhibitor mechanism in 

 the medulla oblongata. 



The fall of general blood-pressure was formerly attributed to a sudden 

 dilatation of the splanchnic blood-vessels alone, in consequence of a depres- 

 sion of that portion of the general vaso-motor center which maintains through 

 the splanchnic nerves a tonic contraction of their walls. It has been satis- 

 factorily demonstrated that this is not the sole cause; for after division of the 

 splanchnic nerves, stimulation of the depressor causes a still further fall of 

 from 30 to 40 per cent, in the general pressure (Porter and Beyer). Evi- 

 dently, not any one, but all portions of the vaso-motor center are subject to 

 the effects of depressor stimulation. 



The Inferior Laryngeal Nerves. Faradization of the inferior laryngeal 

 nerves produces effects which vary in accordance with the strength of the 

 stimulus, with different animals, and with the same animal at different periods 

 of life. In the adult dog and in man, the glottis is kept widely open for 

 respiratory purposes by the tonic contraction of the abductor muscles (the 

 posterior crico-arytenoids) ; for phonatory purposes the glottis is closed and 

 the vocal membranes approximated by the contraction of the adductor mus- 

 cles. It has been shown that these opposed groups of muscles have inde- 

 pendent 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 completely 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 laryngeal 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 stimulation 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 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 med- 

 ulla, the spinal accessory sending no branches to the heart. In the frog 



