510 
MR. SI13SON ON THE MECHANISM OF RESPIRATION. 
and descends to be inserted into the upper edge of the four or five superior costal ribs ; 
the bellies to the respective ribs lie behind each other in laminae : this muscle is much 
shorter in fig. VI. b, inspiration, than in fig. VI. a , expiration, as is another muscle 
arising from the coracoid and inserted into the three first sternal ribs. The fibres of 
the sternal intercostal muscles (9) are much shorter in fig. VI. b, inspiration, than in 
fig. VI. a, expiration ; they, with the sterno-costal muscle, elevate the sternal ribs and 
make them glide on each other. The combined actions of the scaleni, the levatores 
costarum,the sterno-costal muscle, the external intercostals, and the sternal intercos- 
tals raise the ribs, and push the sternum forwards, the vertebrae slightly backwards. 
25. Inspiratory muscles arising from the scapula , Plate XXIV. fig. III. a. 31.32.29. 
I observed the three scapular muscles (31. 32 and 29) to act during inspiration ; 
31 and 32 evidently raise the ribs, acting from the scapula ; 29 acts from the ribs on 
the scapula to elevate it, thus lifting it away from the expanding chest. 
26. Expiratory muscles; internal oblique (18); external oblique (17); rectus (16); 
transversalis (20); internal intercostals (21 and 19 a. figs. III. VI.). 
The antagonist muscles to the dilators of the chest are numerous. The internal 
oblique (18) and the internal intercostal muscles (21), with a muscle (30) acting from 
the pelvis, combine to draw down the spinal ribs, and to cause the lower edges of the 
upper ribs to glide backwards on the upper edges of the lower ribs. The external 
oblique (17), inserted into the spurs, draws the spinal ribs downwards, brings them 
nearer the sternum, and increases the bend of the sternal on the spinal ribs. The 
rectus (16) pulls the sternum with the sternal ribs downwards ; the transversalis (20) 
draws them backwards. In the Fowl the upper part of the sternum is drawn nearer 
the spine, and the three superior ribs are depressed (the third rib owing to its peculiar 
curve) by muscles (19 a.) stretching from the sternum to the ribs. 
2 7. The lungs and the costal walls are more developed in the Stormy Petrel (fig. V.) 
than in any other bird I have figured ; the Swan (fig. VI.) ranks next in costal 
development, then the Hawk (fig. IV.), and lowest of all the Fowl (fig. III.). The 
development of the lungs and of the costal mechanism evidently depends on the 
power to sustain flight, to swim, to dive, or to act energetically. The Fowl, which 
scarcely flies, requires comparatively little lung ; its alimentary canal is protected by 
an enormous prolongation downwards of the sternum, a very trifling portion of which 
serves for the costal articulations. 
MAMMALIA ( not including Man), Plate XXVI. figs. VII. VIII., Plate XXVII. figs. 
IX. X., Plate XXVIII. figs. XI. XII. 
28. The chest is expanded in the Mammalia, as in Birds, by the varying position 
of the ribs ; the principle on which the lungs are dilated in the two classes is the 
same; but there are many important modifications. 
