MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
543 
Other. Diagram C 1.2, figs. VI. a. b. § 18. On inspiration, the angles between the ver- 
tebral and sternal ribs become more open ; the sternum moves forwards and the 
spinal column slightly backwards, and the chest is slightly expanded. These move- 
ments are effected by the combined action of the scaleni and sterno-costal muscles 
on the first vertebral and first sternal ribs respectively ; of the levatores costarum 
and external intercostals on all the lower vertebral ribs ; and of the sternal intercostals 
on all the lower sternal ribs. §§ 19. 20. 23. 24. fig. VI. b. diagram C 2. 
On expiration, the movements are reversed by the internal intercostals, the external 
and internal oblique, recti, transversales, and other muscles. §26, fig.VI.a. diagram Cl. 
In the Mammalia (§§ 28-100. Plates XXVI.-XXIX. figs. VII.-XV*.) the addition 
of a large and efficient diaphragm — that in the Bird is small — modifies the form and 
varies the movements of the ribs and actions of the muscles in different parts. The 
ribs articulate with a dorsal arch (§ 31. X. c.) ; they vary in length, those hinging on 
the centre of the dorsal arch are the longest ; while those on the superior curve gra- 
dually shorten from below upwards ; those on the inferior curve shorten from above 
downwards. Diagram D 1.2. O. 1.2. figs. XIV. a. b. §§ 31. 32. 33. 
By virtue of this arrangement, the ribs, articulating with the superior curve, when 
they are raised, approach each other anteriorly. These ribs have each a cartilage 
articulating with the sternum, that is analogous to the sternal rib of birds ; they 
comprise the thoracic set. § 34. 35 Ass, §§ 67 Dog, 79 Man. 
The ribs that hinge on the inferior curve of the dorsal arch, when raised, recede 
from each other; they have floating cartilages, and are the diaphragmatic set of 
ribs. §§ 34. 35. 6 7- 79. 
Intermediate are the longest ribs connected with the dorsal arch ; their cartilages 
are usually linked together and articulate with the lower end of the sternum ; they, 
when raised, usually recede slightly from each other. Diagram N 2 , fig. XIII. b. All 
the ribs recede from each other behind (Diagram M 2 , fig. XV. b.). The anterior por- 
tions of the ribs move forwards and upwards, and carry with them the costal carti- 
lages and the sternum (O. 2. fig. XIV. b). The posterior portions of the ribs move 
backwards, push backwards the vertebrae, and deepen the dorsal arch. Diagram O, 
fig. XIV. b. 
The scaleni invariably act during the whole time of an inspiration (§§ 47. 58. 82) ; 
and in Man and the Porpoise they expand the portion of lung in the neck (§§ 71. 83), 
assisted in Man by the pleural scalenus (§ 84). They do not draw the cervical ver- 
tebrae to either side, but they draw them, during inspiration, forwards and downwards 
(§§ 72. 82). 
The external intercostals between the thoracic set of ribs are throughout inspiratory 
(§§ 48. 60. 85) ; those portions between their cartilages are expiratory (§ 60) ; between 
the diaphragmatic set of ribs they are inspiratory behind, expiratory to the side and in 
* And numerous additional drawings in the Archives of the Royal Society. 
