682 TEXT-BOOK OF PHYSIOLOGY 



with the exception of the crico- thyroid, which are collectively known as the 

 adductors of the vocal bands. During phonation the adductor muscles over- 

 come the activity of the abductors. With the cessation of phonation the 

 abductors immediately restore the vocal bands to their former respiratory 

 position. 



The activities of these two antagonistic groups of muscles are under the 

 control of the central nerve system. The only pathway for the excitator 

 nerve impulses is through the fibers of the inferior or recurrent laryngeal 

 nerve. The relation of these nerve-fibers both centrally and peripherally, as 

 well as their physiologic action, has been the subject of much experimenta- 

 tion. The results have not always been in accord, owing to the choice of 

 animal, the use of anesthetics, strength of stimulus, etc. 



As the outcome of many investigations it is believed that each muscle 

 group is innervated by its own bundle of nerve-fibers, both of which are con- 

 tained in the inferior laryngeal, though coming from two separate centers in 

 the medulla oblongata. Russell succeeded in separating the fibers for the 

 abductors from the fibers for the adductors in the inferior laryngeal, and in 

 tracing them to their terminations. So completely was this done that it 

 became possible to produce at will, through stimulation, either abduction 

 or adduction, without contraction of the muscle of opposite function. 



The laryngeal respiratory center was located by Semon and Horsley, 

 in the cat, in the upper part of the floor of the fourth ventricle. Stimulation 

 of this area during etherization was followed by abduction of the vocal 

 bands. The efferent fibers of this center are believed by some investigators 

 to leave the central nerve system in the spinal accessory nerve, by others in 

 the lower roots of the vagus. 



From the continuous activity of the abductor muscle, and the stationary 

 position of the vocal bands, it is probable that the medullary center is in a 

 state of continuous activity or tonus, the result probably of reflex influences. 



A cortical representation for laryngeal respiratory movements has been 

 determined by Semon and Horsley in different classes of animals. In the 

 cat especially, stimulation of the border of the olfactory sulcus gives rise to 

 complete abduction of the vocal bands on both sides. The representation is 

 therefore bilateral. 



The phonatory center was located by the same investigators in the medulla 

 near the ala cinerea and the upper border of the calamus scriptorius. Stimu- 

 lation of this area was invariably followed by bilateral adduction of the vocal 

 bands and closure of the glottis. 



A cortical representation for phonatory movements has also been located 

 in the lower portion of the pre-central convolution, near the anterior border. 

 Stimulation of this area gives rise to marked adduction of both vocal bands, 

 indicating that the representation is therefore bilateral. 



Faradic stimulation of the inferior laryngeal nerve during slight ether 

 anesthetization gives rise to closure of the glottis; the same stimulation, 

 however, during deeper anesthetization gives rise to opening or dilatation of 

 the glottis, a fact indicating that either the adductor muscles or their nerve 

 terminals are depressed by the action of the ether before the muscles and 

 nerves of opposite function. The superior laryngeal nerves contain motor 

 fibers for the crico-thyroid muscles. Stimulation of the nerve gives rise to 

 contraction of the muscleand increased tension of the vocal bands. It is 



