RESPIRATION. 



579 



■which by their contraction lead to a decrease in the thoracic capacity, 

 and which thus act as muscles of expiration, are mainly the muscles of 

 the abdomen. When the abdominal muscles' contract, they not only 

 serve to still further force the abdominal organs up into the thoracic 

 cavity, and thus lessen its vertical diameter, but they further serve to 

 pull down the sternum and the middle and lower ribs. By so doing they 

 lessen the antero-posterior and transverse diameters of the thorax. The 

 internal intercostal muscles also probably assist in producing expiration 

 by depressing the ribs. When expiration becomes extremely violent all 

 the muscles are brought into play, which, through their contraction, may 

 either depress the ribs, press on the abdominal viscera, or offer fixed 

 support to muscles having those actions. 



The movements of the thorax in respiration are accompanied by 

 movements of the nostrils and glottis. 



In inspiration the current of air enters through the nostrils, and not 

 by the mouth, and by exposure to the vascular mucous membrane of the 



Fig. 250.— The Human Glottis in a 

 Gentle Inspiration, after Mandl. 

 (Beaunis.) 



I, tongue ; e, epiglottis ; pe, pharyngo-epiglottic fold ; 

 <k, aryteno-epiglottio fold: ph, posterior wall of the 

 pharynx ; c, cartilage of Wnsberg ; ih, superior thyro- 

 arytenoid fold ; ti, inferior fold ; o, glottis. 



Fig. 251.— The Human Glottis in a 

 Forced Inspiration, after Mandl. 

 {Beaunis.) 



6, tip of the epiglottis j g, pharyngo-laryngeal pouch ; 

 I, tongue ; rap, aryteno-epiglottic fold ; ar, arytenoid 

 cartilage; c, cuneiform cartilage; ir, interarytenoid 

 fold ; rn, false vocal cords ; ri, true vocal cords. 



nasal passages b.ecomes warmed up to the temperature of the body. At 

 each inspiration -the external nares expand by the contraction of their 

 dilator muscles ; this movement is especially marked in labored breathing. 

 The horse is incapable of breathing through the mouth, and if the dila- 

 tors of the nostrils be paralyzed, as by section of the facial nerve, 

 asphyxia may be produced. In expiration, the elasticity of the carti- 

 lages of the nostrils is sufficient to cause these parts to return to their 

 usual position. The current of air entering through the nose passes 

 over the passive soft palate to enter the larynx, after passing through 

 the pharynx. At each inspiration the vocal cords are separated and the 

 glottis is thus widely opened (Figs. 250 and 251). At each expiration 

 the arytenoid cartilages approach each other, so approximating the vocal 

 cords, and the glottis is thus narrowed. 



3. The Rhythm op Respiration. — The movements of the column of 



