Mr. Le Gros Clark on the Mechanism of Respiration. 127 



16. The summary of the action of both sets of muscles may be thus 

 stated : — 



(1) They increase the transverse diameter of the chest, by raising the 

 curve of the ribs more nearly to a level with the attachment of the ends. 

 All the fibres must assist in this action, but especially the external inter- 

 costals and the anterior part of the internal intercostals. 



(2) They increase the antero-posterior diameter of the chest by raising 

 the anterior attachmeut of the ribs, and with them the sternum, more 

 nearly to a level with the posterior attachment, thereby also separating 

 the costal cartilages of the lower true and false ribs, and thus widening the 

 interval which separates them on either side of the ensiform cartilage. The 

 posterior fibres of the external intercostals will act with the levatores cos- 

 tarum and (in forced inspiration) with other indirect agents. 



(3) They assist in rotating the ribs outwards, when they elevate them. 



(4) They fix the thoracic walls. This action is a most important one : 

 — a s by giving, when needed, a fixed attachment for the various muscles 

 which are connected to the chest, and which have a double action, accord- 

 ing to the relative mobility of their origin and insertion ; b } by preventing 

 the diaphragm from drawing in the walls of the chest during ordinary 

 inspiration ; c, by resisting the atmospheric pressure during inspiration, 

 by virtue of the tension of the intercostal spaces. 



17. The serratus magnus is usually described as a powerful muscle in 

 forced inspiration, when the scapula is fixed ; but from a careful study of 

 the direction of its fibres, from observation of its action in deep breathing, 

 and (so far as the experiment may be depended on) under the influence 

 of electro-galvanism, I venture to question the correctness of this generally 

 accepted description. The direction of the component parts of this broad 

 muscle would seem to indicate that none but the lowest digitations can 

 assist in elevating the ribs or increasing the capacity of the chest ; on the 

 contrary, the contraction of the upper digitations must rather tend to draw 

 the ribs downwards, and to aid in expiration. It is true that extreme ele- 

 vation of the scapula somewhat alters the relations of the origin and inser- 

 tion of this muscle, but not to such a degree as to impart to it its assumed 

 power, even if such elevation were, as it is not, a necessary condition in 

 forced inspiration. Further, a careful observation of the well-marked 

 digitations of this muscle in a well-developed and thin person, during the 

 act of forced inspiration, failed to discover, either tangibly or visibly, any 

 contraction of their fibres, which was very distinctly marked when the 

 scapula was moved. In stimulation of the muscle by electro-galvanism, 

 though the scapula was freely jerked and moved about, no movement 

 whatever of the ribs or interference with respiration resulted. But it 

 must be admitted that not much importance can be attached to this ex- 

 periment, as the more movable attachment of the muscle would necessarily 

 yield most readily during its contraction. Indeed this remark applies to 

 all experiments of this nature, in which a single muscle is stimulated to 



