MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
519 
48. Intercostal muscles of the thoracic set of ribs. 
By comparing- the views of the Sheep, Pig, and Ass* in expiration with those of 
the same animals in inspiration (the lungs distended), we shall comprehend at a glance 
what muscles are shortened on inspiration, what on expiration ; where the muscles are 
shortest in a, they are expiratory; where they are shortest in b, they are inspiratory. 
During inspiration in the Sheep the first six external intercostals are shortened 
throughout. The superior ribs approach each other more than the inferior, the an- 
terior third of each rib moves forward, the posterior two-thirds are drawn back- 
wards, and the gliding motion of rib on rib is greater behind than before ; hence the 
superior muscles shorten more than the inferior, the posterior fibres more than the 
anterior. The seventh intercostal space widens posteriorly; its muscle is shortened 
behind but lengthened in front, especially where it is inserted into the cartilage, 
which it bends on the rib in expiration. 
During inspiration in the Ass the six superior external intercostal muscles shorten, 
all save the anterior portion of the sixth, and the portions of the fourth and fifth in- 
serted into the fifth and sixth costal cartilages. A great portion of the action of the 
external intercostals is spent in makng the oblique inner surface of the rib more hori- 
zontal. The fibres draw upwards the outer part of the rib, while the inner part is 
stationary, as diagram I 2 . § 4 1 represents, and as is well seen in the third rib of the 
Ass. The gliding motion of the anterior portion of the ribs of the Ass is slightly 
reversed owing to each rib and cartilage forming one piece. The anterior half of 
the five superior conjoined ribs and cartilages moves forwards ; the posterior half of 
those ribs backwards ; the whole of the sixth, -and to a greater extent, the inferior ribs, 
move backwards, though less in front than behind. The sixth, seventh, eighth and 
ninth cartilages either move forward or are stationary. 
The whole of that portion of the internal intercostal between the cartilages shortens 
on inspiration, that part of the muscle having the same action on the cartilages that 
the external intercostal has on the ribs, raising and approximating them, and making 
them glide on each other. The fourth, fifth and sixth muscles between the ribs are 
throughout expiratory. During inspiration, the first, second and third ribs come so 
much nearer to each other in front that the tendency of the fibres to make the lower 
rib glide backwards is counterbalanced : about four-fifths of the first, one-third of the 
second, and a small portion of the third internal intercostal muscles between the ribs 
in their anterior portions are inspiratory ; here the outer and inner layers of inter- 
costals act at the same time by a mutual diagonal pull. 
49. Intercostal muscles of the diaphragmatic and neutral sets of ribs. 
The anterior fibres of the external intercostal muscles between the diaphragmatic 
set of ribs are lengthened, and the posterior fibres are shortened, during inspiration; 
the ribs move further apart, and are all drawn backwards through their whole extent, 
* Fig. VIII. a.b, VII. a.b, X. a.b. Archives of the Royal Society. + Fig. VIII. a. b. ibid. \ Fig. X. a. b. ibid. 
