580 NERVOUS SYSTEM. 



recurrents contain as well motor filaments which preside over movements of 

 the larynx not concerned in the production of vocal sounds. 



The muscles of the larynx concerned in phonation are the crico-thyroids, 

 animated by the superior laryngeals, and the arytenoid, the lateral crico- 

 arytenoids and the thyro-arytenoids, animated by the inferior laryngeals. The 

 posterior crico-arytenoids are respiratory muscles, and these are not affected 

 by extirpation of the spinal accessories, but the glottis is still capable of dilata- 

 tion, so that inspiration is not impeded. If, however, the spinal accessories 

 be extirpated and the larynx be then exposed in a living animal, the glottis 

 still remains dilated, but will not close when irritated. If the inferior laryn- 

 geals be then divided, the glottis is mechanically closed with the iiispiratory 

 act, and the animals often die of suffocation. In view of the varied sources 

 from which the pneumogastrics receive their motor filaments, it is easy to 

 understand how certain of these may preside over the vocal movements, and 

 others, from a different source, may animate the respiratory movements. 



The impediment to the entrance of air into the lungs is a sufficient 

 explanation of the increase in the number of the respiratory acts after divis- 

 ion of both recurrents. The acceleration of respiration is much greater in 

 young than in adult animals. This does not apply to very young animals, in 

 which section of the recurrents produces almost instant death. 



Feeble stimulation of the central ends of the inferior laryngeals, after 

 their division, produces rhythmical movements of deglutition, generally coin- 

 cident with arrest of the action of the diaphragm. These phenomena are 

 generally observed in rabbits, but they are not constant. The reflex action 

 of these nerves in deglutition probably is dependent upon the communicating 

 filaments which they send to the superior laryngeal nerves. 



Properties and Uses of the Cardiac Nerves. The chief uses of the cardiac 

 branches relate to the influence of the pneumogastrics on the action of the 

 heart. This has already been considered in connection with the physiology of 

 the circulation. The effect of dividing the pneumogastrics in the neck is to 

 remove the heart from the influence of its inhibitory nerves ; but at the same 

 time, the operation profoundly affects the respiratory movements, and this 

 latter effect must be eliminated as far as possible in studying the influence 

 of the pneumogastrics on the circulation. The same remark applies to the 

 experiment of Faradization of the pneumogastrics in the neck. The cardiac 

 branches are operated upon with difficulty, and most experiments have been 

 made upon the cervical portion of the pneumogastric itself. 



Faradization of the pneumogastrics in the neck arrests the action of the 

 heart in diastole (the brothers Weber, 1846). This is a direct action and is 

 due to the excitation of the inhibitory fibres, which are derived from the 

 spinal accessory nerves. The phenomena following stimulation of these 

 nerves have already been described in connection with the physiology of the 

 circulation and the properties and uses of the spinal accessories. 



Depressor Nerve. While this nerve, which lias been described in the 

 rabbit (Cyon and Ludwig, 1867), is not isolated in the human subject, it is 

 probable that fibres, the actLon of which is analogous to the action observed 



