CHAP. XXIX.] MUSCLES OF RESPIRATION. 401 



In addition to the muscles now mentioned, the levatores costarum, 

 cervicalis ascendens, and serratus posticus superior, are probably 

 muscles of ordinary inspiration ; and those of the abdominal wall, 

 with the levator ani, of expiration ; the latter action being aided by 

 the elasticity of the ribs and their cartilages, and by the resilience 

 of the elastic tissue entering so largely into the composition of the 

 lungs themselves. This resilience (which Dr. Carson showed to be 

 sufficient in sheep and dogs to balance a column of water from one 

 to one foot and a half in height) occasions the collapse of the lungs 

 when the pleural cavity is accidentally opened, as sometimes by 

 wound of the parietes. In such cases, the air passes in and out of 

 the thorax through the wound in respiration ; and the air previously 

 in the lung is expelled through the glottis by the elastic force of 

 the pleura and walls of the air-passages and cells. If the lung also 

 be wounded, the air may pass into the pleural sac from the air- 

 passages under the same resilient force, and be thence pumped by 

 the expiratory forces through the wound in the parietes into the 

 areolar tissue of the body, as so often happens in the case of frac- 

 tures of the ribs. 



Extraordinary Muscles of Respiration. In voluntary deep 

 breathing, or when (as in asthma) the head, neck, and upper extre- 

 mities become fixed points for the muscles passing between them 

 and the thorax, the lower part of the serratus magnus, the pecto- 

 rales, the subclavii, with the sterno-mastoidei, trapezii, and some 

 others, aid in dilating the chest. Some difference exists among the 

 most recent authors as to the share particular muscles take in the 

 movements of respiration ; and this is not surprising when we con- 

 sider the complexity of the problem, the difficulty of determining 

 the fixed points, or of observing the muscles in separate action. In 

 forced expiration, the triangulares sterni, the serrati postici infe- 

 riores, sacrolumbales, latissimi dorsi, with their accessories as high 

 as the highest costal insertion, probably help to depress and ap- 

 proximate the ribs. 



Power of the Respiratory Muscles. Dr. Hutchinson has lately 

 made numerous experiments on this subject, and his results and those 

 of Valentin and Mendelssohn agree. He finds as the average of 

 1500 trials, that the expiratory power exceeds the inspiratory by one- 

 third ; that men of 5 feet 7 or 8 inches have the greatest inspiratory 

 power, and should on an average raise a column of mercury three 

 inches ; while, above this, the strength gradually decreases as the sta- 

 ture increases, so that a man of 6 feet raises a column of only two 

 inches and a half : also, that occupation or mode of life serves much 



