■>s 



ANATOMY AND PHYSIOLOGY OF 



functions may be exactly the reverse of those of the human muscle whose name 



they earn . 



The tu'o glottic muscles have already been fully described; when both arc cut 

 away or paralyzed, b} section of their nerves, the glottis still closes, owing to the 

 elasticity of its cartilages, but it does not shut firmly, and if the lungs be previously 

 tilled with air, a large part always escapes. Under ordinary circumstances, the 

 glottic lips are closely pressed together by the sphincter-like muscle which we have 

 described and figured. The mass of its fibres lie below the opening muscle, and 

 are parallel to the direction of the glottic Up, while its connections are principally 

 at the anterior and posterior end of the glottic line. When contracted, as it alw a\ s 

 is more or less strongly during the interval between two respirations, it would tend 

 to pucker the glottis somewhat, if it were not that the anterior and posterior 

 insertion are firmly fixed, by the parts in front of and behind them respectively. 

 Thus attached, the only influence it can exert, is to close the glottis whose lips 

 stiffened by the arytenoid cartilages facilitate the process. 



The opening muscle lies outside of the closing muscle, nearly at right angles to 

 it, and immediately under the mucous membrane of the glottic mound. At the 

 moment when expiration begins the respiratory act, this opening muscle contracts 

 so as to draw the glottic lips wide open and permit the air to escape. Then fol- 

 lows a full inspiration, the glottis still open, and lastly it is closed by the constrictor 

 muscle hist after the great flank muscles of inspiration cease to act. 



The downward movement of the hyoid arches is effected by the omo-hyoid and 

 other muscles of the neck. It appears to be intended to remove the glottis from 

 contact with the roof of the mouth during the act of respiration. The upward 

 motion of the hyoid apparatus is produced by a thin sheet of muscular fibres spread 

 transversely across it and over the whole upper part of the neck. 



The function of all of the above muscles was determined by simple observation, 

 by stimulating them directly, and by irritating their nerves. 



The necessity for closing the glottis firmly in these animals becomes obvious, 

 when we reflect, that not only must they be enabled to retain the air, but when 

 under water be competent to exclude that fluid from the lungs. In fact, when we 

 divide the trachea, or in any way paralyze the glottic muscles, the power of retain- 

 ing air in the lungs is totally lost for a time. The moment the respiratory muscles 

 cease to act, the elasticity of the lung asserts itself, and that, viscus is immediately 

 emptied. After a day or two, however, a curious change max lie noticed; the 

 turtle breathes as usual, but in place of allowing the air to escape through the open 

 trachea, the animal holds the inspiratory muscle contracted, and thus retains the air 

 in the lung a considerable time after each inspiration. There seems to be some 

 urgent necessity for thus holding the air a long time in the lung, and perhaps for 

 keeping the Lung distended. The instinctive provision for these purposes when the 

 usual means fail, is well worthy of note. As we proceed with the study of the 

 Laryngeal nerves, we shall have further occasion to observe the great importance of 

 the glottis, and to wonder at the singular means to which creative power has resorted, 

 in order to secure the orifice from the ordinary chances of accident and disease. 

 The physiology of the nerves of respiration in turtles has been the subject of 



