602 TEXT-BOOK OF PHYSIOLOGY. 



to which they are distributed, a partial paralysis of the muscle coat and an 

 impairment of deglutition. 



The Superior Laryngeal Nerve. Faradization of the superior laryngeal 

 nerve gives rise to sensations of pain, and to contraction of the crico-thyroid 

 muscle. Through reflected impulses it causes contraction of the muscles 

 of deglutition, and of the muscles concerned in the act of coughing; inhibi- 

 tion of the inspiratory movement and arrest of respiration in the condition 

 of expiratory standstill, with perhaps a tetanic contraction of the expiratory 

 muscles, and contraction of the laryngeal muscles with closure of the glottis. 

 Peripheral stimulation of this nerve e.g., the contact of foreign 

 particles gives rise to a similar series of phenomena. Division of these 

 nerves is followed by a loss of sensibility in the laryngeal mucous mem- 

 brane, paralysis of the crico-thyroid muscle with a consequent lowering of 

 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 

 crico-ary tenoids) ; 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 com- 

 mon 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 



