536 EESPIRATION 



are given off in the thorax and then return along the trachea to enter 

 the inferior aspect of this organ, they are generally designated as the 

 '' recurrent '' nerves. They are wholly motor in their function and 

 innervate all the laryngeal muscles with the exception of the crico- 

 thyroids. Obviously, therefore, the excitation of this nerve on 

 either the right or left side, must cause a contraction of the muscles in 

 the corresponding half of the larynx, with the exception of the one 

 just mentioned. Accordingly, the division of one or the other of these 

 nerves must lead to a unilateral motor paralysis of this organ, and the 

 division of both nerves, to a bilateral paralysis. Inasmuch as these 

 nerves conduct only in the direction from the center to the larynx 

 and are, therefore, efferent in their function, the excitation of their 

 distal ends must give rise to a contraction of all the laryngeal muscles, 

 with the exception of the cricothyroids. For the same reason, the 

 stimulation of their central ends cannot influence the respiratory 

 rate or amplitude. 



The Function of the Vagus Nerve. — The preceding discussion 

 pertaining to the superior and inferior laryngeal branches of the vagus, 

 must lead us to suspect that the cervical portion of the main trunk of 

 this nerve embraces afferent as well as efferent respiratory fibers. 



FiQ. 264, — Stethographic Record of the Respiratory Movements (Dog) After 

 Division of the Left (LV) and Right (RV) Vagi Nerves. 



This assumption may be tested experimentally by simply dividing 

 one or both nerves above or below the points of origin of their superior 

 laryngeal branches. In either case, this procedure is followed almost 

 immediately by a reduction in the frequency and an increase in the 

 depth of the respiratory movements. The individual movements 

 become pronouncedly inspiratory in their character, and more so, 

 if both nerves have been divided. This change, however, does not 

 necessarily give rise to a dyspneic condition of the animal, because 

 the amount of air furnished by these slow and deep respirations, 

 is practically the same as that previously supplied by the more fre- 

 quent and shallow movements. It is true, however, that the division 

 of both vagi nerves renders the animal incapable of adjusting itself 

 to different conditions. Thus, if it is made to inhale air containing 

 a large percentage of carbon dioxid, it fails to compensate, owing to 

 its inability to increase its respiratory frequency. Working, therefore, 

 on so small a margin, its pulmonary ventilation soon becomes in- 

 adequate for the relief of the high carbon dioxid tension of the blood. 



