RESPIRATION. 



335 



especially in the female, by the scaleni muscles. 

 When the respiration becomes hurried or more 

 laboured, the diaphragm and the muscles that 

 elevate the ribs not only act more vigorously 

 in inspiration, but numerous other muscles, 

 which may be termed auxiliary muscles of 

 inspiration, act in unison with these.* In 

 cases of great dyspnoea, as in a fit of asthma, 

 the shoulders are fixed, the head is thrown 

 back, and all the auxiliary muscles of inspi- 

 ration are brought into violent action. When 

 the shoulders are fixed by the action of the 

 levatores anguli scapulae, the rhombodei ma- 

 jores et minores, and the humeri also fixed by 

 the scapulo-humeral muscles, or by the person 

 grasping some fixed object by the hands, then 

 the muscles, or portions of them which pass 

 between the thoracic extremities and the an- 

 terior and lateral walls of the chest, as the 

 serrati magni, the pectorales minores et ma- 

 jores, the subclavi, and perhaps the costal 

 portion of the latissimi dorsi, act as muscles 

 of inspiration, by pulling the ribs upwards 

 and outwards ^ ; and when the head, cervical 

 vertebrae, hyoid bone, and larynx are fixed by 

 the numerous muscles capable of performing 

 this action, then the sterno-cleido-mastoidei, 

 the sterno-hyoid, and sterno-thyroid muscles, 

 may aid the scaleni muscles in drawing the 

 superior part of the thorax upwards.^ The 

 serrati postici superiores, and the cervicales 

 descendentes, are also accessory muscles of 

 inspiration, if the former be not, at times, in 

 fact a muscle of ordinary inspiration. The su- 

 perior aperture of the larynx is dilated during 

 inspiration by the crico-arytenoidei postici 

 muscles when the breathing is in the least 



that both sets are muscles of expiration, while the 

 latter maintains the more probable opinion, that 

 they act differently in different parts of the thorax. 

 Dr. Hutchinson has also lately made some obser- 

 vations on the actions of these muscles in Medico- 

 Chirurgical Transact, of London, vol. xxix. p. 213. 



[Dr. Hutchinson regards the external intercostals 

 and the intercostilaginous portion of the internal in- 

 tercostals as muscles of inspiration, while the rest of 

 the internal intercostals are muscles of expiration. 

 See a further exposition of this author's views in 

 the article THORAX. ED.] 



* According to Dr. Hutchinson (Opus cit. p. 187) 

 the chief enlargement of the thoracic cavity in deep 

 inspiration is made by the ribs, and not by the 

 diaphragm. 



f^Part of the muscles passing between the thoracic 

 extremities and the anterior and lateral Avails of the 

 chest, here enumerated among the accessory muscles 

 of inspiration, may, in certain cases, act as accessory 

 muscles of expiration, by drawing the scapulae for- 

 cibly downwards upon the ribs. (Vide observa- 

 tions of Mr. Sibson and MM. Beau and Maissiat.) 

 These authors are not of the same opinion regarding 

 the action of all these muscles ; for, while the two 

 former class the serratus magnus among the muscles 

 of inspiration (Opus cit. torn. iii. p. 268. 1843), the 

 latter affirms that the greater portion of its fasciculi 

 acts visibly in violent expiration (Opus cit. p. 535) : 

 they agree, however, in placing the latissimus dorsi 

 among the accessory muscles of expiration. 



J The hyoid bone, larynx, and trachea are some- 

 times drawn downwards during violent inspirations 

 by the strong contractions of the sterno-hyoid and 

 sterno-thyroid muscles, causing a depression of 

 these parts, at the same time that they elevate the 

 sternum. 



hurried ; and in laboured breathing the nos- 

 trils are expanded by the contraction of the 

 muscles, which draw the alae of the nostrils 

 outward. The greater or less demand for 

 fresh air in the lungs regulates the number 

 of these accessory respiratory muscles brought 

 into play, and the energy of their contraction. 



A diminution of the capacity of the thorax 

 or an act of expiration, by which part of the 

 air is expelled from the lungs, follows imme- 

 diately each inspiratory movement. In ordi- 

 nary respiration, after the muscles of inspi- 

 ration have ceased to contract, the elasticity 

 of the thoracic walls, especially of the carti- 

 laginous portion, causes it to return to the 

 state in which it was before its dilatation ; 

 and when the contracted diaphragm has re- 

 laxed, the elasticity of the parts displaced by 

 its descent, is sufficient, without much, if any, 

 aid from the abdominal muscles, to push the 

 diaphragm again upwards. The gas present 

 in greater or less quantity in the digestive 

 tube, being compressed during the descent of 

 the diaphragm, will, from its elasticity, assist 

 in pushing upwards the relaxed diaphragm.* 

 In more forcible expirations, when the walls 

 of the chest are compressed beyond the state 

 they assume when the muscles of inspiration 

 are relaxed, the compressing muscles expe- 

 rience considerable resistance from the elas- 

 ticity of the walls of the chest. 



When the expirations are performed more 

 forcibly than ordinarily, the diaphragm is 

 pushed up, and the sternum and ribs de- 

 pressed by the contractions of the three broad 

 muscles of the abdomen, by the recti abdo- 

 minis, and by the triangularis sterni muscles. 

 The levator ani, one of the antagonist 

 muscles of the diaphragm, assists also in 

 pushing the abdominal viscera upwards. In 

 hurried or laboured expirations the diaphragm 

 is pushed more forcibly upwards by the 

 muscles mentioned, and the ribs are pulled 

 downwards, and the chest compressed, by the 

 quadrati lumborum, serrati postici inferiores y, 

 sacro-lumbales and longissimi dorsi muscles. 



MM. Beau and Maissiat J have described 

 three kinds of ordinary respiratory move- 

 ments : 1. the abdominal, in which the abdo- 

 minal walls chiefly act : 2. the costo-inferior, 

 in which the movements chiefly take place in 

 the lower ribs, from the seventh inclusive, 

 downwards : 3. the costo-superior, in which 

 the superior part of the chest is carried up- 

 wards by the elevation of the superior ribs 

 and the sternum. The first kind, or the ab- 

 dominal type, is observed in infants up to the 

 end of the third year in both sexes ; but after 

 this period the costo-superior type in girls, 

 and the costo-inferior and abdominal types in 

 boys, generally prevail, and this difference 

 becomes more marked as they advance in 

 years. Almost all men, therefore, breathe by 



* Maissiat, in his Etudes de Physique Animale, 

 and Beau and Maissiat in Arch. Ge'ne'r. de MeU 

 torn. iii. p. 263. 1843. 



f Dr. Hutchinson informs us that the body is 

 considerably shortened during violent expiration. 

 (Op. cit. pp. 191, 192.) 



$ Archiv. Gen. de MeU torn. xv. p. 399. 1842. 



