Mr. Le Gros Clark on the Mechanism of Respiration. 131 



for fixing the first rib in inspiration ; and it may be distinctly felt so 

 acting in a thin neck, during forced inspiration, when the rib into which 

 it is inserted is raised with the sternum. The scalenus posticus has a 

 similar action on the second rib. The pectoralis minor, when the coracoid 

 process is fixed, and some few other unimportant muscles, may assist in the 

 elevation of the ribs. In the absence of fixation of the upper or lower ribs, 

 if such condition ever exist, it is difficult to resist the conclusion that the 

 action of the intercostals must necessarily limit the capacity of the chest, 

 by drawing the ribs towards each other and rendering the intercostal spaces 

 rigid. 



Of the lower ribs, experiment has satisfied me that both the tenth and 

 eleventh are raised during deep inspiration — the latter in a less degree than 

 the former. The intercostal spaces between the tenth and eleventh, and 

 between the eleventh and twelfth ribs, are consequently widened consider- 

 ably. The last rib is probably fixed by the attachment of the quadratus 

 lumborum : this rib feels prominent and fixed during deep inspiration ; 

 whereas, under ordinary circumstances, it is loose and not easily felt. 



With the view of ascertaining the mobility of the sternum, and of com- 

 paring the antero-posterior and lateral movements of the chest, I procured 

 a large pair of callipers, with a fixing screw attached. I may thus state 

 briefly the result of my observations on healthy individuals.. 



22. There is no perceptible increase in the antero-posterior diameter of 

 the chest in ordinary inspiration. The antero-posterior diameter of the 

 chest is augmented perceptibly in forced inspiration at the junction of the 

 second rib with the sternum, and but slightly more so at the junction of 

 the sixth rib with the sternum. The mean of the experiments gives about 

 § of an inch, the variation being from § to §• between a state of rest and 

 that of deep inspiration. (An explanatory diagram was here shown.) 



The difference between expiration and a state of rest, in either of the 

 above measurements, is about the same as between rest and inspiration, the 

 latter being somewhat in excess, especially at the lower true ribs. 



The lateral diameter of the chest, in deep inspiration, is augmented, 

 absolutely as well as relatively, to a greater degree than the antero-poste- 

 rior, in the proportion of 6 or 7 to 5, opposite the fifth or sixth, rib; but 

 the maximum increase appears to be attained at about the eighth rib, 

 where the mean is as 10 to 5. 



The movements of the sternum follow those of the costal cartilages, 

 and are dependent on them. These movements, which occur in forced 

 inspiration only, consist in an alternate advance and sinking of the bone, 

 which is most marked at its lower extremity. 



23. The first rib is certainly capable of elevation during inspiration, and 

 the anterior and middle scalenus muscle performs this office : probably its 

 agency is limited, in ordinary inspiration, to fixing the rib. In forced in- 

 spiration, the action of the clavicular portion of the sterno-mastoid is first 

 apparent, and subsequently that of its sternal portion. 



